Nanocoatings and ultra-thin films - Technologies and applications

Nanocoatings and ultra-thin films

© Woodhead Publishing Limited, 2011

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© Woodhead Publishing Limited, 2011

Nanocoatings and ultra-thin films Technologies and applications

Edited by Abdel Salam Hamdy Makhlouf and Ion Tiginyanu

Oxford

Cambridge

Philadelphia

New Delhi

© Woodhead Publishing Limited, 2011

Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 191023406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Control Number: 2011934932 ISBN 978-1-84569-812-6 (print) ISBN 978-0-85709-490-2 (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Toppan Best-set Premedia Limited, Hong Kong Printed by TJI Digital, Padstow, Cornwall, UK

© Woodhead Publishing Limited, 2011

Contents

Contributor contact details Introduction Part I

Technologies

1

Current and advanced coating technologies for industrial applications A. S. H. Makhlouf, Max Planck Institute of Colloids and Interfaces, Germany Introduction Electro- and electroless chemical plating Conversion coatings Chemical and physical vapor deposition (CVD and PVD) Spray coating Other coating techniques New lightweight materials Trends in environmentally friendly coatings, self-assembling and self-cleaning coatings Trends in nanocoatings New composite and powder coatings Advanced polymers and fillers Developments in coating processes Acknowledgements References

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 2

2.1 2.2 2.3

Nanostructured thin films from amphiphilic molecules J. Y. Park and R. C. Advincula, University of Houston, USA Langmuir monolayer Amphiphilic polymers Dendrons and dendrimers

xi xv 1 3

3 4 5 6 7 10 12 13 14 16 17 18 20 20

24

24 28 36 v

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2.4 2.5 2.6 2.7 2.8

Metal/semiconductor nanoparticles 2-D arrays of colloidal spheres Conclusions Acknowledgements References

3

Chemical and physical vapor deposition methods for nanocoatings I. V. Shishkovsky, P. N. Lebedev Physics Institute of the Russian Academy of Sciences, Russia Substrate preparation for ultra-thin films and functional graded nanocoatings Paradigm of functional graded layer-by-layer coating fabrication Nanocoating fabrication methods Physical vapor deposition-based technologies Chemical vapor deposition-based technologies Conclusion and future trends References

3.1 3.2 3.3 3.4 3.5 3.6 3.7 4

4.1 4.2 4.3 4.4 4.5 4.6 5

5.1 5.2 5.3 5.4

Surface-initiated polymerisation for nanocoatings V. Harabagiu, L. Sacarescu, A. Farcas, M. Pinteala and M. Butnaru, ‘Petru Poni’ Institute of Macromolecular Chemistry, Romania Introduction Physisorption and chemisorption, equilibrium and irreversible adsorption Preparation of surface-bound polymer layers Properties and applications Acknowledgement References Methods for analysing nanocoatings and ultra-thin films D. M. Bastidas, M. Criado and J.-M. Bastidas, National Centre for Metallurgical Research (CENIM), CSIC, Spain Introduction Electrochemical methods Surface-sensitive analytical methods for ultra-thin film coatings Spectroscopic, microscopic and acoustic techniques for ultra-thin film coatings

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57 60 61 63 71 74 75 78

78 79 87 110 112 112

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131 132 140 145

Contents 5.5 5.6 5.7

Conclusions Acknowledgements References

Part II Applications 6

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7

7.1 7.2 7.3 7.4 7.5 7.6 7.7 8

8.1 8.2 8.3 8.4 8.5 8.6 8.7

Conventional and advanced coatings for industrial applications: an overview A. S. H. Makhlouf, Max Planck Institute of Colloids and Interfaces, Germany Introduction Conventional coating technologies for the automotive and aerospace industries Advanced coating technologies for the automotive and aerospace industries Packaging applications Coatings for the electronics and sensors industry Paints and enamels industry Biomedical implants industry Acknowledgements References Nanocoatings for architectural glass J. Mohelnikova, Brno University of Technology, Czech Republic Introduction Spectrally selective glass Dynamic smart glazings Glass surface protections Conclusion Acknowledgements References Nanocoatings and ultra-thin films for packaging applications A. Sorrentino, University of Salerno, Italy Introduction Nanomaterials in packaging High barrier packaging Anti-microbial packaging Nanosensors in packaging Packaging as a drug carrier and for drug delivery Nanotechnology solutions for the packaging waste problem

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159 159 162 170 171 173 174 175 177 182

182 183 188 194 195 196 196 203 203 208 209 215 216 218 219

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8.8 8.9 8.10 8.11

Anti-static packaging applications Regulation and ethical issues in the new packaging industry Future trends References

9

Advanced protective coatings for aeronautical applications M. G. S. Ferreira, M. L. Zheludkevich and J. Tedim, University of Aveiro, Portugal Introduction: corrosion in aeronautical structures Types of corrosion in aircraft Factors influencing corrosion Corrosion of aluminum and its alloys Corrosion of magnesium alloys Protective coatings in aerospace engineering Pre-treatments Anodizing coatings Functional nanocoatings in aerospace engineering Nanocoatings for detection of corrosion and mechanical damage Self-healing coatings: nanostructured coatings with triggered responses for corrosion protection Application of nanomaterials for protection of aeronautical structures Conclusion and future trends References

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 10

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11

Nanoimprint lithographic (NIL) techniques for electronics applications I. Tiginyanu, V. Ursaki and V. Popa, Academy of Sciences of Moldova, Republic of Moldova Lithography techniques and nanoimprint lithography (NIL) fundamentals Thermoplastic and laser-assisted NIL Photo-assisted nanoimprinting Soft NIL Extensions of soft NIL Scanning probe lithography (SPL) Edge lithography NIL for three-dimensional (3D) patterning Combined nanoimprint approaches Applications Conclusions

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235

235 236 241 243 244 246 247 253 258 259 261 266 270 270

280

280 286 291 297 301 307 309 311 315 317 320

Contents

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10.12 10.13

Acknowledgement References

322 322

11

Ultra-thin membranes for sensor applications I. Tiginyanu, V. Ursaki and V. Popa, Academy of Sciences of Moldova, Republic of Moldova Introduction Graphene and related two-dimensional (2D) structures Nanometer-thick membranes of layered semiconductor compounds Ultra-thin membranes of gallium nitride Conclusion Acknowledgement References

330

Nanocoatings for tribological applications S. Achanta and D. Drees, Falex Tribology NV, Belgium and J.-P. Celis, Katholieke Universiteit Leuven, Belgium Introduction Use of nanostructured coatings in tribology Review of nanostructured coatings for friction and wear applications Advanced techniques for characterizing tribological properties of nanostructured coatings Conclusions and future trends Acknowledgements References

355

Self-cleaning smart nanocoatings J. O. Carneiro, V. Teixeira, P. Carvalho, S. Azevedo and N. Manninen, University of Minho, Portugal Introduction: TiO2 photocatalysis Photocatalysis processes The photocatalytic cleaning effect of TiO2-coated materials New and smart applications of TiO2 coatings Conclusions References

397

Index

414

11.1 11.2 11.3 11.4 11.5 11.6 11.7 12

12.1 12.2 12.3 12.4 12.5 12.6 12.7 13

13.1 13.2 13.3 13.4 13.5 13.6

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355 356 367 382 391 391 392

397 399 402 406 410 411

Contributor contact details

(* = main contact)

Chapters 1 and 6

Editors

A. S. H. Makhlouf Max Planck Institute of Colloids and Interfaces Department of Interfaces, Am Mühlenberg 1 14476 Potsdam-Golm Germany E-mail: abdelsalam.makhlouf@ mpikg.mpg.de

A. S. H. Makhlouf Max Planck Institute of Colloids and Interfaces Department of Interfaces, Am Mühlenberg 1 14476 Potsdam-Golm Germany E-mail: abdelsalam.makhlouf@ mpikg.mpg.de I. Tiginyanu Academy of Sciences of Moldova Chisinau Republic of Moldova and Technical University of Moldova Chisinau Republic of Moldova E-mail: [email protected]

Chapter 2 J. Y. Park and R. C. Advincula* Department of Chemistry and Department of Chemical and Biomolecular Engineering University of Houston Houston Texas 77204-5003 USA E-mail: [email protected]

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Contributor contact details

Chapter 3

Chapter 7

I. V. Shishkovsky P. N. Lebedev Physics Institute Russian Academy of Sciences Samara branch Novo-Sadovaja st. 221 443011 Samara Russia E-mail: [email protected]

J. Mohelnikova Faculty of Civil Engineering Brno University of Technology Veveri 95 602 00 Brno Czech Republic E-mail: [email protected]

Chapter 8 Chapter 4 V. Harabagiu*, L. Sacarescu, A. Farcas, M. Pinteala and M. Butnaru ‘Petru Poni’ Institute of Macromolecular Chemistry 41A Aleea Grigore Ghica Voda 700487 Iasi Romania E-mail: [email protected]

Chapter 5 D. M. Bastidas, M. Criado and J.-M. Bastidas* Department of Surface Engineering, Corrosion and Durability National Centre for Metallurgical Research (CENIM), CSIC Avda. Gregorio del Amo, 8 28040 Madrid Spain E-mail: [email protected]

A. Sorrentino Department of Industrial Engineering University of Salerno via Ponte Don Melillo I84084 Fisciano - SA Italy E-mail: [email protected]

Chapter 9 M. G. S. Ferreira*, M. L. Zheludkevich and J. Tedim Department of Ceramics and Glass Engineering CICECO University of Aveiro Aveiro, 3810-193 Portugal E-mail: [email protected]

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Chapters 10 and 11

Chapter 13

I. Tiginyanu*, V. Ursaki and V. Popa Academy of Sciences of Moldova Chisinau Republic of Moldova

J. O. Carneiro*, V. Teixeira, P. Carvalho, S. Azevedo and N. Manninen Department of Physics University of Minho 4800-058 Guimarães Portugal E-mail: [email protected]

and Technical University of Moldova Chisinau Republic of Moldova E-mail: [email protected]

Chapter 12 S. Achanta* and D. Drees Falex Tribology NV Wingepark 23B Rotselaar 3110 Belgium E-mail: [email protected] J.-P. Celis Katholieke Universiteit Leuven Dept. MTM 3001 Leuven Belgium

© Woodhead Publishing Limited, 2011

Introduction

Ultra-thin films and nanocoatings play a major role in many areas such as micro- and nanoelectronics, machine building, car and aircraft manufacturing, robotics, etc. Nanocoatings, in particular, represent the interface between the product and the environment and therefore determine not only aesthetic aspects of goods, but also important specific properties such as, for example, anti-corrosion, self-cleaning, chemical and scratch resistance, etc. The term ‘nanocoatings’ is usually used when the coating is nanostructured or its thickness is in the nanometer scale. Nanostructuring is usually applied because of its ability to increase hydrophobicity, radiation hardness, and corrosion resistance and because it makes materials much more flexible. Ultra-thin films and nanocoatings represent two-dimensional (2D) systems, i.e. free electrons in conductive systems can propagate only in the x–y plane. Confinement in the z-direction may add many specific characteristics, especially in the case of electronic materials. Properly designed ultra-thin films and nanocoatings are sometimes used to reduce stiction and light reflection, for surface modification in extreme conditions, and to enhance dirt release properties. Nowadays there is increasing interest in nanophase thermal barrier coatings that exhibit extremely low thermal conductivity. A great deal of attention is also paid to decorative nanocoatings based on special paints and inks. There are various methods of producing ultra-thin films and nanocoatings: vacuum deposition, thermal spray, electrochemical deposition, etc. Vacuum deposition can be based on evaporation, sputtering, thermal decomposition, etc. Among thermal spray methods, one can mention plasma spray and arc spray as well as the high velocity oxygen fuel thermal spray process that provides high density coatings with unique performances in aggressive wear and corrosive environments. The most widely used industrial coating processes are, in fact, based on electroplating and electroless plating. These approaches are relatively simple and cost-effective and are applicable for a wide variety of coatings. xv © Woodhead Publishing Limited, 2011

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Introduction

Currently, nanotechnology enables the production of ultra-thin films and nanocoatings consisting of just one monolayer or a few atomic layers. Such ultra-thin films may functionalize the surface to support desired chemical interactions, or, in contrast, passivate the surface to make it chemically inert. Note that the formation of a few-atomic-layer thick native oxide on the surface of many semiconductor materials is an example of surface passivation. Nanotechnology revolutionizes the application of nanocoatings in many fields, especially taking into account the potential to fabricate nanocoatings with specially designed nanoarchitecture, e.g., nanocomposite polymerbased coatings comprising networks of metal nanodots, aligned metal nanorods, nanowires, or nanotubes, etc. The occurrence of phenomena related to surface plasmon-polariton excitation and negative refraction opens new opportunities for the development of novel focusing optical elements with super-resolution. The discovery of graphene (one-atom-thick sheet of carbon) can be considered as an important breakthrough in the development of nanocoatings. Graphene possesses excellent electrical conductivity and therefore is a unique material for anti-electrostatic applications. Over the last years, researchers have succeeded in fabricating a few-atoms-thick membranes of BN, MoS2, Bi2Te3, Bi2Se3 and GaN. Besides obvious applications in microand nanoelectronics, nanomembranes of GaN seem to be promising for spintronic and biomedical applications. When choosing the type of nanocoatings and the technological approaches for their fabrication, it is very important to take into account their possible impact on the environment. As a rule, increasing investment is made in technologies that are characterized by high efficiency-to-cost ratio and at the same time are environmentally friendly. There is no doubt that in the near future a new generation of multifunctional nanocoatings will be developed with flexible characteristics controlled, in particular, by the conditions of the environment (temperature, pressure, intensity of illumination, etc.). Nanocoatings and ultra-thin films is both a reference and a tutorial for understanding the most common thin-films and coating techniques. The book encompasses recent approaches and future trends in coating and thinfilms technology, looking at essential innovations in the development of industrial nanocoatings and ultra-thin films based on new findings resulting from basic and applied research in the fields of both physics and chemistry. The goal of this book is to discuss the basics of ultra-thin films and nanocoatings and their synthesis techniques, surface characterization, and performance for possible industrial applications. It addresses important questions frequently posed by end-user design engineers, coaters, and coatings suppliers in their quest for multifunctional and superior coating

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qualities for industrial applications. Therefore, contributions in this book emphasize thin films, self-healing coatings, self-cleaning coatings, super-hard nanocoatings, corrosion, tribological and nano-ceramic and nanocomposite coatings with respect to their mechanical and physical properties. Chapter 1 addresses the most common coating techniques. It includes recent developments and future trends in coatings technology and considers the essential innovations in the development of industrial coatings. The chapter highlights future improvements in coating processes based mainly on reduction of the number of coating layers; full automation of the coating process; controlling the end product color through a module method and automatic quality control. Chapter 2 discusses the nanostructuring of thin films of amphiphilic macromolecules and nanomaterials at the air–water interface. The chapter introduces several synthesized amphiphilic materials which have been recently used in the Langmuir–Blodgett (LB) technique. The surface chemistry and properties of the synthesized amphiphilic materials at the air– water interface are also described. Examples of thin film applications using LB film are discussed. Chapter 3 provides a comprehensive analysis of vacuum deposition methods for nanocoating and the production of functional graded (FG) multilayers. A general approach of FG layer-by-layer synthesis is based on a paradigm of the type of connectivity of the internal structure. The objective of the chapter is to demonstrate the particularities and versatility of PVD, CVD, laser-, electron-, and ion-assisted technologies in the engineering of FG nanocoatings with control microstructure. The chapter also provides a description of the nanoperspectives of FG thin films and surface structures with nanoelectromechanical systems (NEMS) properties. Chapter 4 discusses surface-initiated polymerization for nanocoatings. In this chapter, thin polymer layer–surface conjugates are proposed as appropriate materials for studying surface/interface physicochemical properties and material interactions with the environment, allowing performance control over the entire system. Recent advances in surface-attached polymer layers are presented, and thermodynamic and kinetic aspects of polymer physi- and chemisorption are discussed. The chapter also summarizes the preparation methods for polymer-grafted surfaces with the emphasis on controlled processes able to achieve polymer surfaces meeting well-defined criteria. A comparison between the unique properties of polymer brushes and the bulk characteristics or the physisorbed layers is highlighted. Chapter 5 reports the most common and advanced methods for characterization and surface-sensitive analysis of nanocoatings and ultra-thin films. A correlation between the linear potential sweep and impedance measurements for copper specimens under different tarnishing treatments is discussed. The changes in the dielectric constant caused by water

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absorption and the pigment/polymer proportions and porosity of the organic coatings are described with a reasonably good approximation using electrochemical methods. These coatings are characterized by different analytical techniques such as AFM, XPS, infrared, Raman and Mössbauer spectroscopies, x-ray diffraction, ion spectroscopy, glow discharge optical emission spectroscopy, electronic microscopy, scanning acoustic microscopy, and Kelvin probe force microscopy. Chapter 6 provides an overview of conventional and advanced coatings for industrial applications and describes the role of coating technologies in some important industrial applications. The chapter also presents a critical review of recent research and development work on advanced coatings such as smart coatings, ‘super’-hard coatings, and multifunctional coatings, ... etc. The most important aspects of coating technologies for the automotive industry and for sensing, packaging, and biocompatible applications are discussed. Chapter 7 provides a general overview of the main types of nanocoatings for architectural window glass. Glass plays an important role in building design because of its influence on thermal and visual comfort in buildings. Highly transparent coatings are deposited onto architectural windows to be employed in commercial and residential buildings for the purpose of saving energy for heating and air conditioning. They offer environmental benefit because they reduce heat loss and allow passive solar heat gain, reducing the energy consumption required to heat a building as well as energyrelated CO2 emissions from buildings. Chapter 8 discusses the challenges of nanocoatings and ultra-thin films for packaging applications. Packaging technology is of strategic importance as it can be a key to competitive advantage in the modern industry. An innovative pack design can open up new distribution channels, providing a better quality of presentation, enabling lower costs, increasing margins, enhancing brand differentiation product safety and integrity, and improving the logistics service. Thus, there is a persistent challenge to provide cost-effective pack performance, with health and safety being of paramount importance. At the same time, there is a continuous legislation and political pressure to reduce the amount of packaging used and packaging waste. The chapter reports a variety of polymers currently used in packaging and the most widely used plastics in flexible packaging. It also reports different designs and processing techniques used to produce packaging products. Chapter 9 deals with conventional coating technologies and smart nanocoatings for corrosion protection in aerospace engineering. The types and factors which influence corrosion are reviewed as well as the protective coatings that have been in use or which have shown potential for future applications. Moreover, particular attention is given to functional

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nanocoatings for sensing corrosion, nanostructured coatings which self-heal when either corrosion starts or the corrosivity of the environment becomes critical, and other coating properties important in reducing maintenance costs. The chapter concludes that fundamental and applied research in the area of sensor-based, corrosion active and anti-icing/self-cleaning smart coatings is expected to grow in the near future, contributing to the generation of high performance, added-value products. Chapter 10 discusses nanoimprint lithographic (NIL) techniques for electronics applications. The potential of these techniques to surpass photolithography in resolution, and, at the same time, to allow mass fabrication at a lower cost is highlighted. Current and potential uses of NIL are discussed in such fields as data storage, optical components, image sensors, and phase change random access memory devices. Challenges faced by nanoimprint lithography in becoming a standard fabrication technique are also considered. Chapter 11 addresses some technological approaches for the fabrication of ultra-thin membranes for sensor applications and flexible, stretchable, foldable electronics. The discussion focuses on graphene and 2D sheets of layered compounds. The potential to build multifunctional threedimensional (3D) nanoarchitectures based on 2D graphene hybridized with one-dimensional (1D) semiconductor nanostructures is highlighted. The chapter also reviews the fabrication of ultra-thin GaN membranes of nanometer scale thickness by using the concept of surface charge lithography based on low energy ion treatment of the sample surface with subsequent photoelectrochemical etching. Chapter 12 discusses the use of nanostructured coatings as tribological surfaces for both friction and wear reduction with examples from state-ofthe-art research. The chapter gives a general overview of common friction and wear mechanisms encountered in engineering applications. Moreover, it provides a brief review of methods used to deposit nanostructured coatings on substrates. Different advanced techniques for friction and wear characterization of nanostructured coatings and the scale dependence of tribological properties are discussed. The challenges encountered in extrapolating laboratory experiments to field applications are discussed. Chapter 13 looks at the concept of smart materials/coatings – terms usually applied to materials able to change their properties in response to an external stimulus such as light or temperature. New insight is provided into self-cleaning smart coatings and the chapter expands to cover the major features of the photocatalytic materials developed to date. The chapter also gives a historical overview of TiO2 photocatalysis in order to clarify the fundamental characteristics of the photocatalysis processes which take place on TiO2 surfaces. The electronic processes are also discussed, highlighting the main factors controlling the intensity of light

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absorption by the molecule or substrate. The chapter discusses actual and potential applications of TiO2 photocatalysis in industry and in the development of self-cleaning glass materials, giving some practical examples of the application of TiO2 nanoparticles in environment protection. Abdel Salam Hamdy Makhlouf Ion Tiginyanu

© Woodhead Publishing Limited, 2011

1 Current and advanced coating technologies for industrial applications A. S. H. MAKHLOUF, Max Planck Institute of Colloids and Interfaces, Germany

Abstract: This chapter addresses the most common coating techniques currently in use. Recent developments and future trends in coating technology are discussed, taking into account the essential innovations in the development of industrial coatings. These are based on new findings resulting from basic and applied research in the fields of both physics and chemistry. Key words: nanocoatings, coating processes, coating techniques, composite coatings, trends in coatings.

1.1

Introduction

Coatings have been used for centuries in numerous areas of society. The main function of coatings lies in the protection and decoration of materials, and the extent of their use has broadened with increasing social and industrial development. Gooch1 provided a review of the history of paints and the development of coatings. He claimed that the earliest reported paints originated in Europe and Australia approximately 20 millennia ago. During that period, paints based on iron oxide, chalk or charcoal were applied with the fingertips or with brushes made by chewing on the tips of soft twigs. In 9000 bc, the North American people used their primitive paints in the same manner as their European and Australian counterparts to paint the rock walls of their living quarters with pictures of animals and people. More advanced coating technology based on polymeric coatings and paints was developed in ancient Egypt, and later in Greece, Rome and China. Ancient Egyptians used natural resins and wax to form coatings, and artists employed lacquers based on dried oils to protect their paintings. Although polymeric coatings were traditionally mainly used for the protection of various surfaces, other important applications for this type of coating should also be mentioned. Ancient Egyptian scientists developed a very fine coating technology that showed similarities with nanotechnology. Several theories therefore treat nanotechnology as a re-innovated technology, with 3 © Woodhead Publishing Limited, 2011

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the initial attempts at developing nanoscale coatings carried out by Egyptian and, later, Chinese artists. Nowadays, there are probably a few thousand coating systems, ranging from simple systems based on one or two coating steps to sophisticated systems based on multilayers and complicated instruments. However, most of these have an adverse effect on the environment and, in many cases, do not wholly fulfill the demands of the manufacturing industries or of society. The main driving forces behind the sharp increase in research and development in coatings science and surface technology are: • •

an increase in industry requirements for high performance coatings at relatively low cost; increasing regulatory pressure to reduce the hazardous waste (such as hexavalent chromate and volatile organic compounds (VOC)) produced by coating processes, which results in air and water pollution.

There are several techniques employed for the application of a coating onto a substrate. Coatings may be applied as liquids, gases or solids. The following section describes some of the most common coating technologies for metal and alloy substrates.

1.2

Electro- and electroless chemical plating

The modification of the surface properties of the materials to be coated is one of the most desirable methods of improving corrosion and wear resistance, electrical conductivity or decorative appearance. Historically, the chemical processes of electroplating and electroless plating have always constituted the most common, cost-effective and simple techniques for applying a metallic coating to a substrate. In both cases, a metal salt in solution is reduced to its metallic form on the surface of the material to be coated.

1.2.1 Electrochemical plating In electrochemical plating, the electrons for reduction are supplied from an external source. High reactivity materials such as magnesium alloys can quickly form an oxide layer when exposed to air; this oxide layer must be removed prior to plating. Therefore, finding the appropriate chemical surface treatment to prevent oxide formation during the plating process is one of the major challenges involved in plating processing.2–5 Another potential issue is that the quality of the final coating depends on the materials being plated. As a result, different chemical surface treatment processes must be developed for each material to be coated. Uneven

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5

distribution of current density in the plating bath, resulting in non-uniform coatings, is a further problem with this technique. Electroplating also uses a large amount of electricity which can significantly increase the cost of the plating process.

1.2.2 Electroless chemical plating In electroless chemical plating, the reducing electrons are supplied by a chemical reducing agent in solution or from the material itself. This process does not suffer from the same disadvantages as those noted previously for electroplating and even allows complex shapes to be coated. Another advantage of electroless plating is that second-phase particle such as alumina, carbides or diamonds can be co-deposited during the plating process in order to improve some desirable properties such as wear resistance, hardness or abrasion.4,6–9

1.3

Conversion coatings

Conversion coatings are produced by a chemical or electrochemical reaction at a metal surface, which creates a layer of substrate metal oxides, vanadate, chromates, cerate, molybdate, phosphates or other compounds that are chemically bonded to the substrate surface. Conversion coatings are widely used as low-cost coating processes which are able to protect the metal substrate from corrosion by acting as an insulating protective barrier between the metal surface and the environment.

1.3.1 Chromate conversion coating Chromate conversion coating is the most common type of conversion coating applied to improve the corrosion protection performance of many metals and their alloys, including aluminum, zinc, copper and magnesium. Major reasons for the widespread use of chromating are the self-healing nature of the coating, the ease of application, the high electric conductivity and the high efficiency : cost ratio. These advantages have made them a standard method of corrosion protection. Moreover, they provide the greatest level of under-film corrosion resistance and facilitate the application of further finishing treatment. However, the Environment Protection Agency (EPA) ranks hexavalent chromate as one of the most toxic substances due to its carcinogenic effect and because it is environmentally hazardous as a waste product. As a result of current environmental legislation, along with increasing calls for a total ban on toxic hexavalent chromate in coating processes, many attempts have been made to develop less toxic or

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eco-friendly alternatives. Trivalent chromate was proposed as a possible alternative but proved to be less effective than hexavalent chromate.

1.3.2 Chrome-free conversion coatings In the last few decades, chrome-free conversion coatings based on salts such as cerate, stannate, vanadate, molybdate, silicate and zirconate have been developed. These can provide covalent bonding for strong coating adhesion and can act as a barrier coating, limiting the transport of water to the surface of the material.10–23

1.3.3 Anodizing Anodizing is an electrolytic process which is used to produce a thick oxide layer on the surface of metals and alloys. These films are used to improve corrosion resistance and paint adhesion to the substrate.23 The anodizing process includes the following stages: (i) mechanical treatment; (ii) degreasing, cleaning and pickling; (iii) electropolishing; (iv) anodizing using AC or DC current; (v) dyeing or post-treatment; and (vi) sealing.24 The anodized films formed consist of a thin barrier layer at the metal–coating interface and a relatively thick layer of a cellular structure. Each cell contains a pore the size of which is determined by the type of electrolyte and the experimental conditions. The pore size and density in turn determine the quality of the anodized film.23 Electrochemical inhomogeneity due to phase separation in the material to be coated is one of the main challenges faced in the production of uniform anodic coatings. The presence of flaws, porosity and inclusions from mechanical treatment can also result in uneven deposition which, in turn, can enhance corrosion.25 Another disadvantage of anodizing is that the fatigue strength of the materials to be coated can be affected by localized heating at the surface during the treatment,25 especially in thicker films. Moreover, the anodized film formed is made of a brittle ceramic material that may not have the appropriate mechanical properties to fulfill the requirements of some industrial applications.

1.4

Chemical and physical vapor deposition (CVD and PVD)

1.4.1 Chemical vapor deposition Chemical vapor deposition (CVD) is one of the most common processes used to coat almost any metallic or ceramic compound, including elements,

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metals and their alloys and intermetallic compounds. The CVD process involves depositing a solid material from a gaseous phase; this is achieved by means of a chemical reaction between volatile precursors and the surface of the materials to be coated. As the precursor gases pass over the surface of the heated substrate, the resulting chemical reaction forms a solid phase which is deposited onto the substrate. The substrate temperature is critical and can influence the occurrence of different reactions. There are several types of CVD process, including atmospheric pressure chemical vapor deposition, metal-organic chemical vapor deposition, low pressure chemical vapor deposition, laser chemical vapor deposition, photochemical vapor deposition, chemical vapor infiltration, chemical beam epitaxy, plasma-assisted chemical vapor deposition and plasma-enhanced chemical vapor deposition.

1.4.2 Physical vapor deposition Physical vapor deposition (PVD) is a vaporization coating technique, involving the transfer of material on an atomic level under vacuum conditions. The process is in some respects similar to CVD, except that in PVD the precursors, i.e. the material to be deposited, start out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in gaseous form. The process involves four steps: (i) evaporation of the material to be deposited by a high energy source such as an electron beam or ions–this evaporates atoms from the surface; (ii) transport of the vapor to the substrate to be coated; (iii) reaction between the metal atoms and the appropriate reactive gas (such as oxygen, nitrogen or methane) during the transport stage; (iv) deposition of the coating at the substrate surface. PVD has several advantages including: (i) coatings formed by PVD may have improved properties compared to the substrate material; (ii) all types of inorganic materials and some types of organic materials can be used; (iii) the process is environmentally friendly compared to many other processes such as electroplating. However, PVD has also some disadvantages including: (i) problems with coating complex shapes; (ii) high process cost and low output; (iii) complexity of the process.

1.5

Spray coating

Spray coating is a process in which molten or softened particles are applied by impact onto a substrate to produce a coating. Spray coating techniques are widely used in industry for organic lacquers and for coating irregularly shaped glass and metals.26 Examples of some common spray coating techniques are on the following pages.

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1.5.1 Thermal spraying In the thermal spraying process, melted coating materials are sprayed onto the substrate to be coated. Particles of 1–50 μm are partially melted and accelerated to high velocities by a flame or an arc. The particles deposit onto a surface forming a coating, the quality of which is determined by the oxide content, porosity and adhesion to the substrate. The coating materials are usually heated by electrical or chemical means, and the sprayed material can be metal, ceramic or polymer. One of the main advantages of the thermal spray technique is its ability to provide coatings ranging from 15 μm to a few mm thick for substrates with large surface areas, at a high deposition rate compared with other conventional coating processes such as electro- and electroless deposition, CVD and PVD. Another advantage is the possibility of feeding powders of different coating materials such as ceramics, plastics and composites, or pure metal, and spraying them over the substrate surface.27,28

1.5.2 High-velocity oxygen fuel spraying High-velocity oxy-fuel spraying (HVOF) is a modified version of the thermal spray technique, developed in 1980. In this technique, a mixture of liquid or gaseous fuel in addition to oxygen is fed into a combustion chamber, where these are ignited and react with each other. There are several types of fuels used in HVOF. Gaseous fuels such as hydrogen, natural gas, methane, propane, or liquid fuels such as kerosene are commonly used. The resultant hot gas at high pressure (about 1 MPa) passes through a jet of very high velocity (∼1000 m/s). A powder of the coating materials is injected into the hot gas stream, which accelerates the powder up to 700–800 m/s. The stream of hot gas and powder is directed towards the substrate to be coated. The powder partially melts in the stream, and is deposited over the substrate.26 The resultant coating has a thickness of about 10 mm and is commonly used to improve corrosion and wear resistance.

1.5.3 Plasma spraying Plasma spraying is a coating process in which powders of the coating materials are fed into the plasma jet at around 10 000 K, at which the coating materials melt and are sprayed over the substrate to be coated. Owing to the interaction between the plasma coating materials and the substrate to be coated, several factors affect the final properties of the coating, such as the nature of the coating powders, composition of the plasma gas, gas flow

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rate, energy input, torch geometry, distance from the substrate and final coating/substrate cooling parameters.29

1.5.4 Vacuum plasma spraying Vacuum plasma spraying was developed on the basis of the plasma spray technique, and can operate at relatively low temperatures, ranging from 40–120 °C, thus avoiding thermal damage to some types of coating materials such as polymers, rubbers or plastics. Moreover, this process can induce non-thermally-activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmospheric pressure.

1.5.5 Cold spraying The cold spraying coating technique is broadly based on the same ideas as the HVOF spraying technique, in that high velocity is used in order to enhance the interaction between the coating materials and the substrate to be coated. However, in the cold spraying technique, particles are accelerated to very high speeds by the carrier gas, which is forced through a nozzle. Upon impact, solid particles deform plastically and bond mechanically to the substrate to form a coating. Selecting a high velocity range is an important issue in cold spraying, where the velocity must be sufficient to create bonds between the coating materials and coating substrate. The velocity depends on the properties of the material, powder size and temperature. The first attempt to use the cold spraying technique was carried out in 1990 by a Russian research team who were testing the particle erosion of a target exposed to a high velocity gas steam loaded with fine powder. One major advantage of this technique is the possibility of using soft metals such as copper or aluminum as well as elements with high melting points such as tungsten, titanium and tungsten carbide and cobalt.26 Another advantage is the possibility of using an inert carrier gas such as nitrogen or helium instead of oxygen. The disadvantages of this technique are the low deposition efficiency, and the need to use a very fine powder in order to allow higher velocities, which is industrially unattractive.26

1.5.6 Warm spraying The warm spray technique was recently introduced as a novel modification of HVOF spraying.26 In this technique, the temperature of the combustion gas is lowered by mixing it with nitrogen, which is similar to the principle behind the cold spraying technique.

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The advantage of this technique is that the coating efficiency is higher than when cold spraying is employed. Moreover, the lower temperatures used for warm spraying result in reduced melting and fewer chemical reactions in the feed powder compared to HVOF. These advantages are especially important for coating materials such as titanium, plastics and metallic glasses, which rapidly oxidize or deteriorate at high temperatures.

1.6

Other coating techniques

1.6.1 Sol–gel coatings New developments in the chemical tailorability of mixed alkoxide sol–gel coatings have led to the creation of an environmentally friendly and longlasting conversion coating for many ferrous and nonferrous alloys. Two of the most common problems associated with applying the sol–gel technique to protect metals against corrosion are: (i) the poor adhesion performance of the coatings formed using sol–gel processing; and (ii) the absence of high-performance coating systems based on environmentally acceptable salts. Recent research has shown that coatings based on silica, ceria, vanadia and molybdate can be adapted using sol–gel technology to produce a functionally gradient coating. This coating is able to provide covalent bonding for strong coating adhesion and can act as a high performance surface treatment, limiting water transport-induced attack on the surface of the material.12,13,30–37 This technique was successfully applied with different aluminum alloys.

1.6.2 Spin coating Spin coating has been used for several decades for the application of thin films. In this process, a small drop of the coating material is loaded onto the centre of a substrate, which is then spun at a controlled high speed. In the spin coating process, the substrate spins around an axis which should be perpendicular to the coating area. As a result, the coating material spreads towards, and eventually off, the edge of the substrate leaving a thin film of coating on the surface. Final film thickness and other properties will depend on the nature of the coating (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process such as the rotation speed.

1.6.3 Gravure coating The gravure coating process relies on an engraved roller running in a coating bath, which fills the engraved dots or lines of the roller with the

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coating material. The excess coating on the roller is wiped off by the doctor blade and the coating is then deposited onto the substrate as it passes between the engraved roller and a pressure roller.38

1.6.4 Roll-to-roll coating Roll-to-roll coating is the process of applying a coating to a flat substrate by passing it between two (or more) rollers. In this technique, the coating material is applied by one or more auxiliary rolls onto an application roll after the gap between the upper roller and the second roller has been appropriately adjusted. The coating is wiped off the application roller by the substrate as it passes around the support roller at the bottom. After curing, the coated substrate is then shaped to the final form; this has no effect on the properties of the coating. Roll-to-roll coating is made up of two different techniques: direct roll coating and reverse roll coating. In the direct roll coating technique, the applicator roll rotates in the same direction as the substrate. In the reverse roll coating technique, the applicator roll rotates in the opposite direction to the substrate.38

1.6.5 Knife over roll coating The knife over roll coating process is one of the most suitable coating techniques for high viscosity coatings and rubbers. In this process, the coating material being applied to the substrate passes through a gap between the knife and the roller. The excess coating is scraped off using the knife.38

1.6.6 Air/knife coating The air/knife coating process is similar to the knife over roll coating process. However, a powerful air jet is used instead of the knife. This is an extremely simple process, in which the coating is applied to the substrate and the excess is ‘blown off’ by the air jet; however, the noise associated with the air jet makes the process industrially unattractive.38

1.6.7 Meyer rod coating In the Meyer rod coating process, the coating is applied onto the substrate as it passes over a roller partially immersed in the coating material. The quantity (and sometimes the shape) of the coating to be applied on the substrate is controlled by a wire-wound metering rod (known as the Meyer rod), with the quantity determined specifically by the dimensions of the wire used.38

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1.6.8 Slot/die and slot/extrusion coating In the slot/die process, the coating is squeezed out by gravity or by externally-applied pressure through a slot and onto the substrate surface. If the coating is composed entirely of solids, the process is known as ‘extrusion’. The coating thickness can be controlled by adjusting the line speed compared with the speed of the extrusion.38

1.6.9 Dip coating In the dip coating process, the substrate is immersed into a bath of the coating material of determined viscosity, with withdrawal speed and immersion time carefully controlled. The substrate is then removed from the bath and allowed to drain. The coated substrate can then be dried by force-drying or baking. Dip coating is extremely dependent on the viscosity of the coating. The coating viscosity must remain constant during the coating process. The main use of this process is the application of primers prior to final coats.

1.6.10 Curtain coating In the curtain coating process, a bath with a slot of a determined dimension in its base allows a continuous curtain of the coating to fall into the gap between two conveyors. The coating substrate is passed along the conveyor at a controlled speed so the coating material can be applied at the substrate surface.38

1.7

New lightweight materials

Interest in lightweight materials is increasing both in industry and in research circles. Magnesium alloys are one example of these lightweight materials to replace heavy alloys in the automotive and aerospace industries, resulting in savings in fuel consumption and a reduction in CO2 emissions. Magnesium alloys have a variety of excellent properties, including a high strength-to-weight ratio, low density, dimensional stability and castability. However, despite their excellent mechanical properties, magnesium alloys remain very susceptible to corrosion. Several coating schemes have been proposed to improve the corrosion resistance of magnesium.10–22 However, the existing methods are frequently either expensive, such as PVD and nitrogen ion implantation, or unable to create the surface properties desired for many applications where magnesium alloys would otherwise be highly competitive. On the other hand, an increased demand for new plastics and composite materials will require the development of both new coating solutions and

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new application processes. Innovative changes in manufacturing processes will also require efficient coatings to be developed, which meet the appropriate surface quality standards.39,40

1.8

Trends in environmentally friendly coatings, self-assembling and self-cleaning coatings

This section describes the future trends in environmentally friendly coatings, self-assembling and self-cleaning coating technologies, reviewing the essential innovations in (i) the materials to be coated, (ii) the structure and chemistry of the coatings and (iii) coating techniques, based on new findings from basic research in physics and chemistry.

1.8.1 Environmentally friendly coatings The demand for more environmentally friendly coatings will increase as a result of stricter environmental legislation and tightening regulation on the use of hexavalent chromate and VOCs. The industrial coatings of the future will always be chrome-free, and low in solvent or solvent-free, provided that the resulting properties, both in terms of protection and appearance, are still acceptable. The old days of trial and error formulating with standard binder systems will give way to new binder resins with tailor-made properties and welldefined molecular structures, especially in the paint industry where a broad molecular weight distribution increases the viscosity of the solution. More attention will be paid to the statistical design of functionality in polymers to ensure greater uniformity of crosslinking during curing, and to optimize film performance. A number of monomers will cease to be used due to their toxic potential.39

1.8.2 Self-assembling molecules Self-assembling molecules can arrange themselves regularly and closely on a metal surface, and can then polymerize as a second step. This very flexible, strongly-anchored layer could improve coating adhesion, corrosion protection and mechanical and chemical resistance. The use of natural materials such as oils and resins has declined in paint binders because their performance did not match the desired requirements for modern coatings, but biochemical gene modifications could make natural raw materials attractive as components for paint binders. Natural polymers such as cellulose or chitin can be modified to make them more acceptable in paint binders.39–41

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1.8.3 Self-cleaning coatings Another interesting trend is the development of self-cleaning coatings which are resistant to dirt, water and oil when applied to a wide range of substrates. Micro-patterns on surfaces are known to resist water and dirt penetration, and this so-called ‘lotus effect’ is already used in self-cleaning roof tiles and sanitary ware.39 Bright surfaces on car bodies, however, do not seem to be a realistic goal: artificial lotus surfaces are not self-renewing, so their sensitivity to cleaning and other mechanical treatments presents a problem. Other surface effects such as shark skin and fur-like films might find outlets in the coating of plastics.39

1.9

Trends in nanocoatings

The evolution of nanotechnology is behind the recent dramatic changes in several areas of scientific research and technology. In the area of surface coatings, new approaches that make use of nanoscale effects can be used to create tailor-made coatings with significantly enhanced properties. The ultimate impact of nanotechnology in the area of coatings will depend on its ability to direct the assembly of hierarchical systems that include nanostructures. Future approaches will focus on tailor-made coatings for specific functions, either by incorporating existing identifiable components into the desired coating or by the formation of new structures during the coating process.42 Interest in nanocoatings has increased because of the potential to synthesize materials with unique physical, chemical and mechanical properties. There are several types of design models for nanocoatings, such as nanocomposite coatings, nanoscale multilayer coatings, super lattice coatings, nanograded coatings, etc.43 In the last decade, research interest in nanostructured coatings has increased due to the potential for enhancing the coating functionality for specific applications such as environmentally friendly anti-corrosion coatings for the automotive and aerospace industries.13,15,37 The data showed a significant improvement in the corrosion resistance of materials as compared to materials processed using conventional coating methods. The following section reviews the most common nano-based coating systems and their expected future impact.

1.9.1 Micro- and nanocapsule-based coatings Micro- and nanocapsules or containers are of great interest to both industry and the scientific research community, and have a wide range of applications such as molecular biology, electronic materials, medical imaging and

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photonic crystals. Moreover, they have also been increasingly used as fillers, coatings, capsule agents, etc., because of their low density and optical properties. These shells are created either by in situ hydrolysis of the corresponding metallic salt in the presence of core materials,44–49 or by calcination of polymer particles coated with uniform inorganic shells.41, 50–56 Recently, several nanocontainers were synthesized using a two-step process. In the first step, charged polystyrene nanospheres were prepared using emulsion polymerization or polymerization in suspension. In the second step, the polystyrene lattices were coated using the sol–gel method to form a layer of inorganic oxide(s). The composites were treated in air to burn off the polystyrene latex. Using this approach, different nanocontainers such as cerium/molybdenum oxide, cerium/titanium oxide, iron/titanium oxide, silicon/calcium oxide, polypyrrole and polyaniline were produced.55 However, further research still needs to be carried out in order to optimize the experimental coating conditions such as the coating thickness and temperature. Moreover, a multistep process is industrially unattractive. To make self-repairing coatings, the researchers first encapsulated a catalyst into spheres less than 100 μm in diameter. They also encapsulated an inhibitor or a healing agent into similarly sized microcapsules. The microcapsules are then dispersed within the desired coating material and applied to the substrate.39 When the coating is subjected to corrosion or scratching, some of the capsules break open, spilling their inhibitor contents onto the damaged region. The healing agent reacts with the environment to form a protective oxide to repair the damage, depending upon environmental conditions. Another approach based on a dual-function tailor-made capsule containing a healing agent and a catalyst has also been proposed. The healing agent (inhibitor) offers a self-healing property which protects against scratches and corrosion and the catalysis provides extra functions such as antimicrobial effects or other desired functions.

1.9.2 Nanocomposite coatings A nanocomposite coating consists of a nanocrystalline phase and an amorphous phase. Several techniques have been proposed for the preparation of nanocomposite coatings. However, reactive magnetron sputtering is most commonly used. Nanocomposite coatings can be tailored to offer superior hardness above the maximum given by the rule of mixture (materials with hardness greater than 80 GPa are called ultra-hard materials). Therefore, detailed theories have been developed and extensive experiments carried out with the aim of optimizing the hardness of nanocomposite coatings for many industrial applications. However, while a great deal of attention is

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given to increasing the hardness of a material, insufficient attention is paid to its toughness. Accordingly, further research should be carried out which considers both hardness and roughness while taking into account corrosion behavior. In designing nanocomposite coatings, several factors and application requirements must be considered. Research areas in the field should involve prevention of the formation of dislocations in the nanocrystalline phase; blocking of the grain-boundary sliding of nanograins; the effect of changes made to the lattice parameter; the role of the crystal size; the nature of the grain boundaries; and finally the effects of impurities and intermediate phases.56

1.10

New composite and powder coatings

1.10.1 Composite coatings Composite coating technology has been developed to fulfill the industrial demands for coatings whose specifications exceed the capabilities of conventional coating technologies, and that are capable of functioning in extreme environments and in the face of challenges posed by temperature, corrosion, abrasion, fatigue, friction and erosion.57 Tungsten carbide hard metals and their analogues are now a mature technology; however, recent research has focused on changing the design of the microstructure with the aim of producing alternatives to the conventional two-phase structure. Significant improvements in the performance of coatings have been achieved by changing the size, shape and distribution of the phases to produce ultra fine-grained materials.57 Some approaches involve the use of nanoscale powder to form nanocomposite coatings as discussed in Section 1.9.2.

1.10.2 Powder coatings Designing a high performance clear coating is a key target for the automotive industry. Powder coating offers a limitless choice of colors and finishes. Moreover, powder coating produces a high specification coating which is relatively hard, abrasion-resistant and tough. The thickness of the coating applied can be varied considerably according to requirements. The prospects for powder coatings will improve with the development of materials with lower curing temperatures, and the production of thinner films will become an achievable goal as powder particle morphology improves. The use of UV curable powder coatings seems to have great potential. Controlling the humidity content of the surface can also facilitate electrostatic coating. The size and shape of the powder particle will be important factors in achieving much smoother films. Ultrasonic waves seem

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to be a promising means of adjusting the shape of the powder particle to make it more spherical.

1.11

Advanced polymers and fillers

Developments in the structure of coatings using advanced polymers represent one noteworthy future trend in coating technology.

1.11.1 Hyperbranched polymers Controlled radical polymerization will be used for the synthesis of hyperbranched polymers with low melt viscosities; this is ideal for coatings with a high solid content and for powder coatings. The addition polymerization of acrylic monomers in hypercritical fluids is one potential means of producing solvent-free binders with narrow melting ranges for use in powder coatings with low film thickness and low temperature curing characteristics.39,40

1.11.2 Organic–inorganic hybrid polymers Hybrid polymers have proved to be of great interest in the development of future coating systems. Combinations of organic polymers and silicates make it possible to improve the overall coating qualities, as the stability and scratch resistance of inorganic networks can be combined with the elasticity of organic polymers. The sol–gel method has been successfully used for the synthesis of hybrid polymers.39 Most of the current multiphase polymer systems do not produce translucent films, a fact that has limited their wider acceptance as coatings. A method for producing polymers with improved translucent qualities was reported by Brock.39 This method uses a construction of block and comb polymers with intramolecular incompatibility, followed by phase separation in the nanofield. Future developments in phase separation, along with a better understanding of the interface energies of polymer mixtures, will lead to improved adhesion to the surface with no negative effect on the coating qualities.

1.11.3 Conductive polymers Newly-developed conductive polymers based on polyanilines and polythiophens might offer improved corrosion resistance. These could also permit electrostatic and electrochemical coating of non-metallic substrates, as well as producing heat-conductive layers for electrically heatable surfaces.

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Microporous coatings containing catalysts are currently the subject of extensive research in environmental science, due to their ability to remove toxic gases. Coatings that contain a small amount of active components (such as photochromic coatings, which are translucent or opaque according to light intensity) will be increasingly used as a means of identifying ownership of stolen items.39,40

1.11.4 Water-soluble paint An increase in the use of water-soluble latex paints is one of the biggest future trends in architectural paint industries. Water-soluble paints are less expensive, lower in odor than alkyd-based solvent-borne paints, and produce no toxic waste. In the coatings and paints industries, water-soluble paints have met with strong competition from recently-developed superior resins with unique characteristics.

1.11.5 Fillers Fillers are widely used in every coatings industry, mainly to reduce costs, although they can negatively affect the coating quality. Future developments in the chemistry and structure of the fillers will lead to an increase in their use in tailor-made coating systems; the fillers will then have a specific function such as strengthening the mechanical properties of the coatings or improving the coating quality for specific applications and decorative effects. Nanoscience and nanotechnology will have a significant impact on the design of ultra-thin films containing nanolayers of special fillers or additives embedded into the matrix of the polymer. It is expected that such fillers will improve the mechanical strength, coating transparency and corrosion resistance. Phyllo silicates with their unique leaf-like structure will increasingly be the subject of both industrial and basic research as a filler for tailor-made coatings. Some studies used phyllo silicates interlocked into the polymer matrix in nano form to improve the barrier effect of the coating. This approach will become more popular and will be used for thermal and electrical insulation, and especially for fire protection.39,40

1.12

Developments in coating processes

The processing of nanocoatings will be of interest to researchers due to the superior hardness and strength that these coatings can offer. However, developments in nanocoatings are determined by improvements in coating processing and the availability of nanopowders. The use of nanocoatings is still in its infancy because the process requires large-scale control during

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the synthesis of nanoparticles. Moreover, the technology requires sophisticated instruments and multistep processes. Nanopowders are used as feedstock materials for thermal spray processes (plasma spraying or HVOF spraying). Thermal spraying offers the unique advantage of moderate to high rate of throughput and the ability to coat target materials with complex shapes using nanostructured feedstock powders prepared from vapor, liquid and solid routes. Thermal sprayed nanocoatings with moderate hardness showed better wear resistance than those fabricated by micro powders.58 HVOF will get the most research attention in the next generation of nanocoatings, due to its ability to deposit dense nanocrystalline ceramic coatings with wear properties superior to those produced by plasma spraying, thanks to the lower spraying temperature involved. Moreover, HVOF allows the development of nanocoatings with low porosity, high strength and increased wear resistance. Some modifications will be made to the electro-dip processes in order to allow lower curing temperatures; new curing mechanisms using radiation curing will also extend their field of application to low melting point materials such as plastics.39 Automation of the coating process in order to increase line speed, reduce labor dependency and save energy will be the main target for coating industries. Future improvements in coating processes will include reduction of the number of coating layers; full automation of the coating process; controlling the color of the end product using a module method; and automatic quality control. The future trends in coating technology are generally based on: 1. Advanced lightweight materials such as magnesium alloys and composite materials. Developments in magnesium alloys and composite materials will continue, focusing on rare-earth alloying elements such as cerium and yttrium with the aim of providing some desirable properties. 2. Tailor-made coating systems with unique chemistry and structures will become commonplace thanks to innovations in polymers, new binders and fillers. 3. Nanoscience and nanotechnology will play a distinct role in the next generation of coating technology. More attention will be paid to selfhealing coatings based on nanocontainers or nanocapsules that can be filled with inhibitor to protect the substrate from corrosion upon damage or scratching in the coating layer. However, more studies still need to be carried out to optimize the coating conditions since the technology currently involves multisteps (around 8–10 steps) and expensive raw materials, both of which are industrially unattractive. 4. Many future efforts will be devoted to the design of biocompatible nanocoatings systems for medical implant applications and to the

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development of suitable techniques for preparing high performance hydroxyapatite coatings. 5. Developments in the coating process will be an important topic for industrial and basic research. New surface modification techniques and faster and cheaper curing methods using UV and electrostatic application will be the focus of the next decade of research.

1.13

Acknowledgements

Many thanks to all the authors of papers, books, and websites and to all published sources (listed below) that were used to prepare the materials for this chapter.

1.14

References

1. Gooch Jan W (2006), Lead-Based Paint Handbook, New York: Kluwer, 13–33. 2. Innes W (1974), ‘Electroplating and electroless plating on magnesium and magnesium alloys’, in Schlesinger M and Paunovic M (eds), Modern Electroplating, New York: Wiley-Interscience, 601–617. 3. Hajdu J, Yarkosky E, Cacciatore P and Suplicki M (1990), Electroless nickel processes for memory disks, in Romankiw L and Herman DA Jr (eds), Proceedings of the Fourth International Symposium on Magnetic Materials, Processes and Devices, Pennington, NJ: Electrochemical Society, 90, 685–691. 4. Sharma AK, Bhojraj H, Kaila VK and Narayanamurthy H (1997), ‘Anodizing and inorganic black coloring of aluminum alloys for space applications’, J Metal Finishing, 95, 14–20. 5. Luo J and Cui N (1998), ‘Effects of microencapsulation on the electrode behavior of Mg2Ni-based hydrogen storage alloy in alkaline solution’, J. Alloys Comp., 264, 299–305. 6. Chen J, Bradhurst D, Dou S and Liu H (1998), ‘The effect of chemical coating with Ni on the electrode properties of Mg2Ni alloy’, J. Alloys Comp., 280, 290–293. 7. Wang CY, Yao P, Bradhurst DH, Liu HK and Dou SX (1999), ‘Surface modification of Mg2Ni alloy in an acid solution of copper sulfate and sulfuric acid’, J. Alloys Comp., 285(1–2), 267–271. 8. Ellmers R and Maguire D (1993), A Global View of Magnesium: Yesterday, Today, Tomorrow, Waukonda, IL: International Magnesium Association, 28–34. 9. Gray JE and Luan B (2002), ‘Protective coatings on magnesium and its alloys – a critical review’, J. Alloys Comp., 336(1–2), 88–113. 10. Hamdy AS (2008), ‘The effect of surface modification and stannate concentration on the corrosion protection performance of magnesium alloys’, Surf. Coat. Technol., 203, 240–249. 11. Hamdy AS and Farahat M (2010), ‘Chrome-free zirconia-based protective coatings for magnesium alloys’, Surf. Coat. Technol., 204, 2834–2840. 12. Hamdy AS (2009), ‘The correlation between electrochemical impedance spectroscopy and other polarization techniques for the corrosion evaluation of

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23. 24.

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coated and bare metals in aqueous solutions’, in Kalnin˛sˇ T and Gulbis V (eds), Corrosion Protection: Processes, Management and Technologies, New York: Nova Science Publishers, No. 7, 161–173. Hamdy AS (2010), High Performance Coatings for Automotive and Aerospace Industries, New York: Nova Science Publishers. Hamdy AS (2010), ‘An attempt for designing economically attractive chromefree conversion coatings for magnesium alloys’, in Hamdy AS (eds), High Performance Coatings for Automotive and Aerospace Industries, New York: Nova Science Publishers, 127–138. Hamdy AS (2008), ‘A novel approach in designing chrome-free chemical conversion coatings for automotive and aerospace materials’, Pitture e Vernici (European Coatings), 86(3), 43–50. Hamdy AS (2006), ‘Enhancing corrosion resistance of magnesium alloy AZ91D in 3.5% NaCl solution by cerate conversion coatings’, Anti-Corr. Meth. Mater., 53(6), 367–373. Hamdy AS (2006), ‘Green cerate based surface treatment for improving the corrosion resistance of magnesium alloy AZ91D in marine environments’, Proceedings Green Engineering for Materials Processing Symposium, Materials Science & Technology Conference and Exhibition (MS&T’06), 15–19 October, Cinergy Center, Cincinnati, OH, 141–150. Hamdy AS (2007), ‘Alkaline based surface modification prior to ceramic based cerate conversion coatings for magnesium AZ91D’, Electrochem. Solid-State Lett., 10(3), C21–C25. Hamdy AS and Butt D (2007), ‘Eco-friendly conversion coatings for automotive and aerospace materials’, invited talk, European Coatings Conference ‘New Concepts for Anti-Corrosive Coatings, 14–15 June, Berlin. Marx B, Hamdy AS, Butt DP and Thomsen D (2007), ‘Assessing the performance of stannate conversion coatings on Mg alloys’, Symposium ‘Corrosion and Coatings Challenges in Industry’, 88th Annual Meeting, 17–21 June, Boise, ID, co-located with the 62nd Annual Meeting of the American Chemical Society. Hamdy AS, Marx B, Butt DP and Thomsen D (2007), ‘Novel eco-friendly stannate-based conversion coatings for Mg alloys’, Surface Treatments and Processing Session, Symposium: Automotive and Ground Vehicles: Materials and Processes for Vehicles, MS&T ’07 Conference and Exhibition, 16–20 September, Detroit, MI. Hamdy AS, Marx B and Butt DP (2009), ‘A new approach in designing ecofriendly low cost chemical conversion coatings for magnesium alloys’, Keynote Speaker, 6th International Symposium on Surface Protective Coatings, 25–28 February, Goa, India. Mittal CK (1995), ‘Merbromin as a colouring agent for anodized surfaces of Al and its alloys’, Trans. Metal Finishers Association of India, 4, 227. Kasten LS, Grant JT, Grebasch N, Voevodin N, Arnold FE and Donley MS (2001), An XPS study of cerium dopants in sol–gel coatings for aluminum 2024T3, Surf. Coat. Technol., 140, 11–15. Sharma AK, Uma Rani R and Giri K (1997), Studies on anodization of magnesium alloy for thermal control applications, Metal Finishing, 95, 43–51.

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26. Kuroda S, Kawakita J, Watanabe M and Katanoda H (2008), Warm spraying – a novel coating process based on high-velocity impact of solid particles, Sci. Technol. Adv. Mater., 9, 17. 27. Paulussen S, Rego R, Goossens O, Vangeneugden D and Rose K (2005), Plasma polymerization of hybrid organic–inorganic monomers in an atmospheric pressure dielectric barrier discharge, Surf. Coat. Technol., 200(1–4), 672–675. 28. Leroux F, Campagne C, Perwuelz A and Gengembre L (2008), Fluorocarbon nano-coating of polyester fabrics by atmospheric air plasma with aerosol, Appl. Surf. Sci., 254(13), 3902–3908. 29. Anon. (2010), Plasma spraying process, available at: http://www.zircotec.com/ page/plasma-spray_processing/39 (accessed April 2011). 30. Hamdy AS (2006c), ‘Corrosion protection of aluminum composites by silicate/ cerate conversion coating’, Surf. Coat. Technol., 200(12–13), 3786. 31. Hamdy AS and Butt DP (2006), ‘Environmentally compliant silica conversion coatings prepared by sol–gel method for aluminum alloys’, Surf. Coat. Technol., 201(1–2), 401–407. 32. Hamdy AS (2006), ‘Advanced nano-particles anti-corrosion ceria based sol gel coatings for aluminum alloys’, Mater. Lett., 60(21–22), 2633–2637. 33. Hamdy AS and Butt DP (2006), ‘Corrosion protection performance of nanoparticles thin-films containing vanadium ions formed on aluminum alloys’, AntiCorr. Meth. Mater., 53(4), 240–245. 34. Hamdy AS and Butt DP (2007), ‘Novel anti-corrosion nano-sized vanadia-based thin films prepared by sol–gel method for aluminum alloys’, Mater. Proc. Technol., 181(1–3), 76–80. 35. Hamdy AS (2006), ‘A clean low cost anti-corrosion molybdate based nano-particles coating for aluminum alloys’, Prog. Org. Coat., 56(2–3), 146– 150. 36. Hamdy AS, Butt DP and Ismail AA (2007), ‘Electrochemical impedance studies of sol–gel based ceramic coatings systems in 3.5% NaCl solution’, Electrochim Acta, 52, 3310–3316. 37. Hamdy AS, Shoeib M and Butt DP (2009), ‘A novel approach in designing environmentally compliant sol–gel based ceramic coatings and nanocomposite coatings for industrial applications’, in Malik A and Rawat RJ (eds), New Nanotechniques, New York: Nova Science Publishers, 649–659. 38. Anon. (2010), Technical Coating International Inc., available at: http://www. tciinc.com/coating.html (accessed April 2011). 39. Brock T (2005), ‘Trends in coatings technology’, Pitture e Vernici (European Coatings), 81, 15–24. 40. Murata K (1996), ‘Trends in coatings technology’, J. Macromol. Sci., Part A, 33(12), 1837–1841. 41. Wang M, Jiang M, Ning F, Chen D, Shiyong L and Duan H (2002), ‘Blockcopolymer-free strategy for preparing micelles and hollow spheres: self-assembly of poly(4-vinylpyridine) and modified polystyrene’, J. Macromolecules, 35, 5980. 42. Baer DR, Burrows PE and El-Azab AA (2003), ‘Enhancing coating functionality using nanoscience and nanotechnology’, Prog. Org. Coat., 47, 342–356. 43. Zhang S, Sun D, Fu Y and Du H (2003), ‘Recent advances of superhard nanocomposite coatings: a review’, Surf. Coat. Technol., 167, 113–119.

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44. Hwang YJ, Oh C and Oh SG (2005), ‘Controlled release of retinol from silica particles prepared in O/W/O emulsion: The effects of surfactants and polymers’, J. Control Release, 106, 339–349. 45. Zhang Y, Hu Q, Fang Z, Cheng T, Han K and Yang X (2006), ‘Self-assemblage of single/multiwall hollow CeO2 microspheres through hydrothermal method’, Chem Lett., 35, 944–945. 46. Zhan L and Wan M (2003), ‘Self-assembly of polyaniline – from nanotubes to hollow microspheres’, Adv. Funct. Mater., 13, 815–820. 47. Ocana M, Hsu WP and Matijevic E (1991), ‘Preparation and properties of uniform-coated colloidal particles 6. Titania on zinc oxide’, Langmuir, 7, 2911–2916. 48. Kawahashi N and Matijevic E (1990), ‘Preparation and properties of uniformed coated colloidal particles V. Yttrium basic carbonate on polystyrene latex’, J. Colloid Interface Sci., 138, 534–542. 49. Tapeinos C, Kartsonakis IA, Liatsi P, Danilidis I and Kordas G (2008), ‘Synthesis and characterization of magnetic nanocontainers’, J. Am. Ceram. Soc., 91, 1052–1056. 50. Yang M, Niu Z, Dong X, Xu H, Zhaokai M, Zhaoguo J, Lu Y, Hu Z and Yang Z (2005), ‘Synthesis of spheres with complex structures using hollow latex cages as templates’, Adv. Funct. Mater., 15, 1523–1528. 51. Tartaj P, Teresita GC and Serna CJ (2001), ‘Single-step nanoengineering of silica coated maghemite hollow spheres with tunable magnetic properties’, Adv. Mater., 13, 1620–1628. 52. Wang D, Song C, Lin Y and Hu Z (2006), ‘Preparation and characterization of TiO2 hollow spheres’, Mater. Lett., 60, 77–80. 53. Eiden S and Maret G (2002), ‘Preparation and characterization of hollow spheres of rutile’, J. Colloid Interface Sci., 250, 281–284. 54. Song C, Wang D, Gu G, Lin Y, Yang J, Chen L, Fu X and Hu Z (2004), ‘Preparation and characterization of silver/TiO2 composite hollow spheres’, J. Colloid Interface Sci., 272, 340–344. 55. Anon. (2009), MULTIPROTECT Newsletter, No 2, p. 3, available from: http:// multiprotect.org. 56. Zhang G, Yu Y, Chen X, Han Y, Di Y, Yang B, Xiao F and Shen J (2003), Silica nanobottles templated from functional polymer spheres, J. Colloid Interface Sci., 223, 467–472. 57. Allcock BW and Lavin PA (2003), ‘Novel composite coating technology in primary and conversion industry applications’, Surf. Coat. Technol., 163–164, 62–66. 58. Tjong SC and Chen H (2004), ‘Nanocrystalline materials and coatings’, Mater. Sci. Eng. R 45, 1–88.

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2 Nanostructured thin films from amphiphilic molecules J. Y. PARK and R. C. ADVINCULA, University of Houston, USA

Abstract: Over the past several decades, the Langmuir monolayer and Langmuir–Blodgett (LB) techniques have been widely utilized for the characterization of small amphiphilic molecules such as organic polymers (i.e. block copolymers, star block copolymers, and dendritic polymers), inorganic nanomaterials, and even carbon nanotubes and biomolecules. This chapter will introduce several amphiphilic and colloidal materials and describe their surface chemistry and properties at the air–water interface. Finally, some examples of thin film applications using LB films will be discussed. Key words: Langmuir monolayer, Langmuir–Blodgett (LB), Langmuir– Schaefer (LS), air–water interface, phase transition.

2.1

Langmuir monolayer

Thin insoluble or amphiphilic monolayers at the air–water interface have been studied for more than 200 years since Benjamin Franklin characteristically made the first observation in 1774. Later, in 1890, Lord Rayleigh1 reported a quantitative measurement on a monolayer of oil molecules resulting from spreading on water. In 1891, Agnes Pockels2 applied a simple equipment to observe general behavior of surface tension with varying surface concentrations of oil and to measure the thickness of films of various amphiphillic substances on the surface of water. Then, in 1917, Irving Langmuir further improved the technique that was already used by Pockels.3 With this new device he confined the film between a fixed barrier on one side and a floating one on the other side, and the monolayer was oriented on the water surface via the compressing floating barrier (Fig. 2.1). The surface pressure was recorded by measuring the actual force on the floating barrier. Since then, a ‘Langmuir monolayer’ has been defined as a molecularly thin layer trapped at the air–water interface.4 Typically, small insoluble, amphiphilic molecules possess a polar or charged hydrophilic head group, e.g. -COOH and -NH3, and a hydrophobic hydrocarbon tail (-CH3(CH2)n, with n > 4).5 24 © Woodhead Publishing Limited, 2011

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2.1 A schematic representation of a Langmuir–Blodgett rough being compressed.

dc

I

Barrier

2.2 Isotropic liquid film in a trough with a movable barrier. The arrows show the direction of the barrier’s movement.

2.1.1 Surface pressure The surface pressure (π) of a film is obtained from the difference between the surface tension of the pure subphase, γ0 (pure water) and that of the subphase with monolayer, γ. That is: π = γ0 − γ

[2.1]

For a simple isotropic system as shown in Fig. 2.2, a thin film is on a pure water surface. If the barrier is moved back by dx (i.e. the film area is stretched by dA = ldx), the change of surface free energy, G, is ΔG = γldx

[2.2]

where the surface tension γ is the free energy per surface area (unit: mN/m) and l is the length of the barrier. If the barrier is moved slowly so that the temperature is not changed and the force acting on the barrier is invariant, the force should be γl. The work on the system while stretching the film is γldx. Also, the surface tension γ could be considered as the force per length.

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2.1.2 Surface pressure isotherm A surface pressure isotherm is one of the major experimental techniques to examine the phase behavior of quasi-two-dimensional systems on air– liquid interface. Depending on the balance between hydrophilicity and hydrophobicity, the molecules may be more or less soluble in water. When amphiphilic molecules between the two barriers are spread on the water surface, the molecules are far apart and lie on the surface randomly. This is a two-dimensional (2-D) gaseous (dilute) phase, shown as ‘G’ phase in Fig. 2.3. Upon compression from a gaseous phase, the monolayer undergoes several phase transitions. As the barriers move towards each other, the molecules become closer, resulting in increasing interactions among them. Phase separation into a ‘gas’ and a ‘liquid’ occurs on the monolayer. After that, the monolayer behaves like a two-dimensional liquid. This disordered liquid, a ‘liquid expanded phase’ corresponds to ‘LE’ in Fig. 2.3. Further

Liquid expanded (LE) Untilted condensed

60

Gaseous (G)

Surface pressure (mN/m)

50 (LE+G) 40

200

Tilted condensed

400

(LC)

30

Liquid expanded 20

(LE) (LC+LE)

10

Phase coexistence: condensed+liquid expanded 20

25

30 35 Area per molecule (Å2)

40

2.3 A typical surface pressure isotherm for a Langmuir monolayer after reference 7. The insets in the figure show the molecular orientation at the air–water interface. The surface pressure is from reference 8 measured in pentadecanoic acid.

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compression of the monolayer gives rise to a transition from liquid expanded to a condensed phase with a plateau indicating a first-order transition.6 The molecules become more compact and they also begin to orient with some order in the hydrocarbon chains (a ‘liquid condensed phase’, i.e. ‘LC’ phase in Fig. 2.3). With further compression, all the molecules go over into an untilted condensed phase without any tilt angle. This is a typical surface pressure isotherm of a fatty acid. If the layer is compressed further, the monolayer will break and the molecules will form three-dimensional (3-D) structures of various orientations (it is called ‘collapse’).4

2.1.3 Langmuir trough and other experimental techniques A Langmuir trough is the main instrument to spread amphiphiles from an organic solvent solution and generate thin monolayers at the air–water interface. Instead of polymer coated troughs used in the early stages of development, metal or glass troughs with a wax coating were used. These days, a Teflon trough is the most commonly used for troughs because it is hydrophobic and chemically inert. While compressing or expanding the molecules on the surface with Teflon barriers, the effect of the monolayer on the surface pressure of the liquid is measured through use of a Wilhelmy balance, or electronic wire probes. A Langmuir–Blodgett (LB) film can then be transferred to a solid substrate by vertical dipping the substrate through the monolayer or by ‘picking-up’ the monolayer through a flat substrate in a horizontal dipping method, the so-called Langmuir–Schaefer (LS) method. Various methods and devices can be used in conjunction with the trough in order to investigate in-situ the properties of thin films. These methods include Brewster angle microscopy (BAM) to observe reflectivity images in real time and x-ray reflectivity to measure the diffraction behavior of a deposited film, the Wilhelmy Plate to measure the surface pressure, and a surface potential meter to measure the surface potential. Usually, a water bath is used to control the temperature and a robotically controlled dipper is used to transfer the LB films onto a solid substrate, i.e. whilst maintaining a surface pressure or area while controlling the deposition of alternating LB or LS monolayers. The transfer of a monolayer to a substrate is a sophisticated process which is dependent on many factors, such as the direction and speed of a substrate, surface pressure, temperature, and pH of the subphase not to mention the stability of the insoluble monolayer or amphiphile for transfer. A dipping holder with the substrate can be programmed to pass through the interface from top to bottom or bottom to top at a set speed. Depending on the hydrophilic or hydrophobic substrate, the dipping process can start from below the liquid surface or above the liquid surface, respectively. Multilayers can be achieved by sequential dipping through alternating monolayers.

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2.1.4 Recent research related to Langmuir monolayers and Langmuir–Blodgett (LB) While the Langmuir monolayer and the LB technique have been utilized mostly for characterization and deposition of small amphiphilic molecules, they have been highly useful tools to determine equilibrium and dynamic behavior in thin monolayers at the air–water interface, related to phase transition (from gas phase to liquid and solid phase) of all types of materials in general. Recently, these techniques have been extensively used for characterization and application of a variety of polymers (i.e. block copolymers, star block copolymers, and dendritic polymers), inorganic nanomaterials, and even carbon nanotubes and biomolecules. Based on these techniques, molecular interaction, molecular dynamics, and molecular orientation at the air–water interface, reflecting morphological changes and phase transition behaviors, can be more precisely analyzed with a combination of other instrumental techniques. The rest of the chapter introduces several synthesized amphiphilic macromolecules and nanomaterials which have been investigated with the LB technique. Their physico-chemical properties at the air–water interface will be described. Finally, some examples of applications using the LB film method will be discussed.

2.2

Amphiphilic polymers

2.2.1 Block copolymers Block copolymer self-assembly at the air–water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution. Thus, the interfacial behavior and surface morphology of aggregates of block copolymers on the water surface using the LB technique can be used to fundamentally understand the nature of polymeric materials at interfaces. Since year 2000, various synthesized polymers have been extensively introduced and investigated for their unique properties and organization at the air–water interface. Polystyrene (PS) is a very common anchoring block because it is nonpolar and forms tightly clustered aggregates when spread at moderate concentrations under ambient conditions on the water surface. Recently, poly(styrene)–b–poly(ethylene oxide) [PS–b–PEO] copolymers have been extensively studied. Predominantly hydrophobic PS–b–PEO LB films have three types of features: dots (circular aggregates), spaghetti (rod-like aggregates), and continents (planar aggregates), upon change in concentration. This parameter directly controls a competition between the film spreading and polymer entanglement during solvent evaporation (Fig. 2.4a).9 The other parameter is control of ratio of the PEO blocks. By increasing the

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29 Air

Compression High conc. Low π

High π

Water

Compression Low conc. (a) Solvent evaporation Concentration increase

NPS >> NPEO PS

PEO

NPS << NPEO (b)

2.4 Surface morphological behavior of PS–b–PEO diblock copolymer depending on (a) concentration and (b) relative ratio of the block lengths.

ratios of the PEO blocks in two block copolymers [MW = 141K (11.4 wt % PEO) and MW = 185K (18.9 wt % PEO)] with varying spreading solution concentrations (0.1–2.0 mg/mL), a variety of interesting aggregates (spaghetti, dots, rings, and chain-like aggregates) were observed. This is because the degrees of PS chain entanglements in the spreading solution result in different kinetic self-assemblies at the air–water interface.10 The micellization of the copolymer at the air–water interface was modeled for predicting the size and aggregation number of circular surface micelles (Fig. 2.4b).11 In contrast, dendritic polystyrene–graft–poly(ethylene oxide) (PS–g–PEO) copolymers form different surface morphologies depending on the content of the PEO blocks.12 At low surface pressures (gas phase), the PEO chains remain adsorbed at the air–water interface whereas at higher surface pressures (condensed phase), the PEO chains partially desorb into the subphase (brush-like conformation). The copolymer aggregates are observed as large islands (<15% PEO content) or ribbon-like superstructures (22–43% PEO content) upon compression. Seo et al. have described molecular weight (MW) dependent surface morphological behaviors of polystyrene–b– poly(methyl methacrylate) (PS–b–PMMA) diblock copolymer.13 Moreover, polystyrene–block–poly(N-isopropylacryamide) (PS–b–PNIPAM) diblock copolymer monolayers at the air–water interface have revealed that the

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surface pressure changes with temperature and compression rate strongly affects the conformations of PNIPAM chains at the interface.14 On the other hand, ionization of repeating units on block polymers by (i) changing pH or (ii) adding ionic salts in subphase provides another approach to changing interfacial behavior and morphology of the Langmuir monolayer. (i) The solubility of neutral vinylpyridine units on polystyrene– b–poly(vinylpyridine) (PS–b–PVP) copolymers is controlled by pH, resulting in varying degrees of desorption of the blocks into the water subphase. Thus, the surface micelles of the PS–b–PVP copolymer form different surface morphologies at the air–water interface depending on pH change.15 At high pH (basic conditions), where the degree of ionization is low, the π-A isotherm shows a more expanded curve and high transition surface pressure due to the strong interaction of P2VP blocks spreading on the water surface. As the pH is lowered, the transition surface pressure decreases while the transition region becomes more extended, indicating equilibrium between the floatation and submergence of the P2VP blocks upon surface pressure change. However, at pH 1.8 (acidic conditions), where the 2VP units are completely ionized, such blocks are submerged into the water and the transition behavior is not observed. Thus, the π-A isotherm behavior can be understood by considering the balance of the solubility and the electrostatic repulsion of the ionized P2VP chains. Based on the balance of the hydrophobic attraction of PS cores and the electrostatic repulsion of submerged P2VP chains, the LB films of the PS–b–P2VP surface micelles show isolated circular micelles at high pH. As the intermicellar distance becomes shorter, and the micelles eventually make contact with each other, a laced network of circular micelles at low pH was observed. Analogous studies have been performed using poly(styrene–b–ferrocenyl silane) (PS–b–FS)16 or 3-pentadecylphenol17 mixture with PS–b–PVP. In these systems, interesting morphological behaviors were observed, such as the formation of a dense network of interconnected nanostrands and nanostructured patterns under external electric fields, respectively. As a similar system to PS–b–PVP copolymer at low pH, strongly ionized amphiphilic diblock copolymers of poly(styrene)–b–poly(styrenesulfonate) (PS–b–PSS) with various hydrophilic and hydrophobic chain lengths were studied for their interfacial properties and self-assembly behavior with various analytical techniques.18 Matsuoka et al. described interfacial behavior of various ionic amphiphilic diblock copolymers at the air–water interface, such as poly(hydrogenated–isoprene)–b–poly-(styrenesulfonate) (PIp–h2–b–PSS),19poly(1,1-diethylsilacyclobutane)–b–poly(methacrylic acid) (PEt2SB–b–PMMA).20 The ‘critical brush density’ of ionic blocks was determined either from a ‘carpet’ layer or a ‘carpet + brush’ structure formed at the air–water interface as a function of the brush density and salt concentration by mainly using in-situ x-ray reflectivity (Fig. 2.5).

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Nanostructured thin films from amphiphilic molecules Carpet+brush

31

Carpet–only

+Salts

+Salts

2.5 Illustration of the interfacial behavior (carpet and brush or only carpet layer) of ionic block copolymers, controlled by ionic salts.

Recently, amphiphilic dendronized block copolymers have also been investigated in terms of their interfacial behavior and surface morphology. The surface properties of two well-defined dendrimer-like copolymers based on PS and poly(tert-butylacrylate) (PS–b–PtBA) or poly(acrylic acid) (PS–b–PAA) were introduced by Duran et al.21 These structures are composed of a PS core with 2, 4, 6, or 8 arms and a corona of PtBA with 4, 8, 12, or 16 arms, respectively. Langmuir monolayer studies of PSnPtBA2n showed the formation of stable aggregates of circular surface micelles observed at around 24 mN/m. Beyond this surface pressure, the monolayers collapsed on the interface, resulting in the formation of large and irregular desorbed aggregates. Additionally, the interfacial self-assembly of the PS–b–PAA was examined at various subphase pH values. This behavior is similar to that of an AB diblock copolymer with ionized blocks. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS–b–PAA sample becomes highly water-soluble. The sample does not form stable monolayers, but instead irreversibly dissolves in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS–b–PAA is less water-soluble and becomes surface-active. The pseudo plateau observed

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in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. Atomic force microscopy (AFM) images revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudo plateau region is reflected by the gradual aggregation of the micellar PS cores above the PAA chains. The surface behavior of an asymmetric heteroarm PEO/PS star polymer at the air–water interface on a solid substrate has also been investigated.22 These amphiphilic star polymers with different numbers of hydrophobic arms and a similar hydrophilic block were synthesized considering different structural features, such as four- and three-arm molecules, PEO–b–PS3 and PEO–b–PS2, the length of PS chains, and the number (three and two) of PS arms. Functional terminal group combinations (i.e. bromine, amine, hydroxyl, and carboxylic terminal groups) for (X-PEO)2–(PS-Y)2 heteroarm star copolymers, with respect to their interfacial behavior and surface morphology, have been included as well. Hydrophilic end-functional groups attached to hydrophobic chains and hydrophobic end-functional groups attached to hydrophilic chains resulted in the stabilization of the spherical domain morphology. Matmour et al.23 recently described the preparation and the surface properties of monolayers of a new set of (PB–b–PEO)4 amphiphilic four-arm star block copolymers. Isotherm experiments at the air–water interface showed three characteristic regions: a compact brush region, a pseudo plateau, and a pancake-like region. The monolayer exhibited reproducible hysteresis at different pressures and demonstrates different morphologies from analogous (PS–b–PEO)4 star copolymers.

2.2.2 π-conjugated polymers Since the 1990s, π-conjugated polymeric materials have been developed for electronic applications through the tuning of their photophysical properties resulting from variation in their chemical structures. These synthesized polymeric materials with desirable tunability and properties can be used for various applications, including electroluminescent displays, solar cells, field-effect transistors (FET), memory devices, and sensors.24 The LB technique is one of the most effective and precise methods for controlling the organization and characterizing the properties of these polymer films at the nanoscale for device fabrication. Recently, Valiyaveettil et al. investigated asymmetrically substituted poly(paraphenylene) with hydrophilic and hydrophobic side chains at the air–water interface using the LB technique (Fig. 2.6).25 They have also constructed Langmuir–Schaefer (LS) monolayers and Langmuir–Blodgett–Kuhn (LBK) multilayers of conjugated poly(p-phenylene)s (CnPPPOH) with alkoxy groups of different

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Nanostructured thin films from amphiphilic molecules

n

O (CH2)n CH3 (a)

O (CH2)n CH3

O (CH2)n

50

OH

40 30 20

(b)

30

1st cycle 2nd cycle

20 10 0 10

20 30 40 Area per repeat unit (Å2)

10 (a) 0 0

CH3

Surface pressure (mN/m)

OH

Surface pressure (mN/m)

OH

33

10

20

30

40

50

Area per repeat unit (Å2)

2.6 (a) Chemical structure of poly(p-phenylene) derivative and (b) π-A isotherm and hysteresis curve (inset) of C6PPPOH. (The isotherm is adapted from reference 25(b))

chain lengths (C6H13O-, C12H25O-, and C18H37O-) and hydroxyl groups on the polymer backbone (Fig. 2.3). The polymer with a short alkoxy chain (C6PPPOH) forms uniform Langmuir monolayers at the air–water interface compared to longer alkoxy chains due to none or minimum hindrance of the short chain for effective transfer of monolayers. Various poly(p-phenyleneethynylene) derivatives were also designed and studied to reveal the relationship between the spatial arrangement and the photophysical properties of fluorescent polymers in thin films with controlled structures. The interfacial behavior and configuration (edge-on or face-on) of the polymers at the air–water interface, depending on the selection of certain functional groups (hydrophilic or hydrophobic) on the polymer backbones, play an important role in determining the photophysical properties of these LB films.26 Moreover, there have been very few reports on the fabrication of LB film using phenylenevinylene (PPV) derivatives.27 Poly[2-methoxy, 5-(n-hexadecyloxy)–p–phenylenevinylene] (MH–PPV) has been investigated to examine the influence of long alkyl side chains on the organizational behavior of Langmuir films at the air–water interface and multilayer structure.28 The adjacent conjugated main chains of MH–PPV in LB films are aligned in a side-by-side parallel fashion and packed with the plane of the π system approximately perpendicular to the layer plane (H-aggregate structure). The long alkyl side chains are characterized by an all trans-zigzag conformation, and the LB films of MH–PPV are well ordered with layer-by-layer structure and a d-spacing of 49.9 Å. Similarly, Langmuir films have been prepared from poly[(2-methoxy-5-n-hexyloxy)–p–phenylenevinylene](OC 1OC6–PPV) (Fig. 2.7).29 The stability and the area per monomer in the condensed film region indicate the formation of monolayers with a very small extent of

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Nanocoatings and ultra-thin films H H H

H C C H H

C

H C H

C H

H H

C H H O H C Air

C O

C H H H

H

Water

n

2.7 A possible arrangement of poly(phenylenevinylene) derivatives at the air–water interface. (This figure is from reference 29)

aggregation, which is attributed to the linearity of the alkyl side chain. The LB films have distinctive features due to the organization of the interpolymers. The application of the LB technique to π-conjugated polymers offers a unique approach for constructing molecular devices. For example, thin film FETs by the LB technique were made from regioregular poly(3-hexylthiophene) (RR-PHT) (Fig. 2.8a).30 The Langmuir films of the RR-PHT are stable at the air–water interface, where polymer backbones adopt an edge-on conformation, and can be transferred onto hydrophobic substrates by horizontal deposition. Thin semiconducting films consisting of regioregular poly(3-hexylthiophene) (RR-PHT) and poly(N-dodecylacrylamide) (pDDA) were also constructed by the LB technique.31 The polymer thin film, chemically doped by contacting with FeCl3 in acetonitrile solution, has a conductivity of 5.6 S/cm and shows the semiconducting properties of FET with mobilities of 2.2 × 10−4 and 4.4 × 10−4 cm2 V−1 s−1. In addition, an efficient organic electrochemical transistor (OECT) composed of this polymer LB film was fabricated. The mixed-polymer LB film composed of 10 layers was used as the conduction channel layer in the OECT. The OECT had an on/off ratio of 1.1 × 104 and mobility of 7.5 × 10−2 cm2 V−1 s−1 at low gate (VG = −1.2 V) and source-drain voltages (VDS = −0.5 V). The relatively high on/off ratio and low charge consumption in this OECT are suitable for macroelectronic device applications. On the other hand, Reitzel et al.32 have utilized the LB technique for fundamentally understanding the self-assembly of various RR-PT derivatives at the air–water interface, related to the structural orientation and

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2.8 Polythiophene derivatives. (a) π-A isotherms of regioregularpoly(3-hexylthiophene) with two concentrations of 0.5 mg/ml and 0.25 mg/ml. (b) Schematic representations of poly(3′-dodecyl-3-(2,5,8-trioxanonyl)-2′,5-bithiophene) at the air–water interface. (These data and the figure are adapted from references 30 and 32)

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Nanocoatings and ultra-thin films

molecular interaction (Fig. 2.8b). These conjugated polymers self-assemble as 2-D polycrystalline monolayers at the air–water interface with the amphiphilic polymers pre-organized into rigid boards standing edge-on at the water surface. The monolayer consists of highly ordered (∼70% crystalline) domains, with a centered rectangular unit cell having the polymer backbone along the a-axis and the thiophene π-stack along the b axis with a distance of 3.85–3.94 Å, depending on the surface pressure.

2.3

Dendrons and dendrimers

As a new class of macromolecules, dendrimers and dendrons are one of the most promising materials class due to their unique geometric architectures and properties. Since the 1990s, considerable effort has been devoted to the study of the behavior of dendrimers (including dendrons) at surfaces and interfaces.33 The highly compact, globular shape, as well as the uniform size and functionality of dendrimers, make them ideal molecular building blocks for a wide range of interfacial materials involving self-assembled monolayers, Langmuir films, multilayers, layer-by-layer (LBL) films, and other assemblies. Moreover, the study of the interfacial behavior of dendrimers on the water surface provides an insight into their unique chemical and physical properties. The dendrimer self-assembly in thin films involves equilibrium macromolecular interaction and dynamics related to the interdistance and density of the terminal functional groups at the air–water interface. Herein, several typical amphiphilic dendrimer or dendrons are introduced, such as (i) poly(benzyl ether), (ii) poly(amidoamine), and (iii) poly(propylene imine), etc. Also various physical and chemical factors, such as phase isotherms, kinetic stability, and molecular configuration, investigated through different surface analytical techniques, were utilized to probe the nanoscale molecular behavior, molecular interactions, and molecular assembly of these dendrimers at the air–water interface.

2.3.1 Poly(benzyl ether) The interfacial behaviors and molecular interactions of dendrimers and dendrons on the water surface were initially studied by Saville et al.34 Two generations ([G-4]-OH and [G-5]-OH) of convergent Fréchet-type poly(benzyl ether) dendrons with hydrophilic hydroxyl groups at the focal point were compared in their work. The π-A isotherm is strongly dependent on the molecular weight (MW). The [G-4]-OH shows a peak collapse transition from a liquid-expanded phase to a liquid-condensed phase due to a nucleation and growth process. For the [G-5]-OH, the surface pressure is steeply increased without a transition behavior. In a more recent work, the π-A isotherms of a series of dendrons ranging from [G-2]-OH to [G-5]-OH

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represented by the lower generation ([G-2,3, and 4]-OH) dendrons have surfactant like behavior, while the higher generations ([G-5, and 6]-OH) exhibited non-surfactant like behavior.35 Later, Kirton et al. and Kampf et al. and Frank performed further studies with methyl ester- or cyanidesubstituted dendrimers at the periphery or dendrimers with hexa(ethylene glycol) on a core (Fig. 2.9), respectively.36 The orientation of hexagonal cylinders, self-assembled from an asymmetric fan-shaped second generation dendrimer containing the CO2C3H7 core group of the cylinder and dodecyl (C12H25) alkyl tails, at the air–water interface is strongly affected by surface anchoring.37 From the π-A isotherm and BAM measurements, the molecule forms a stable monolayer on the water surface with two phase transitions. The condensed monolayers clearly reveal that the molecule forms edge-on oriented monolayers with hexagonal packing on the water surface, suggesting fractional cylindrical configuration at the air–water interface. Upon further compression, the condensed monolayer is transformed into a multilayer, and the interfacial structure adapts a planar morphology. Similarly, an asymmetric, fan-shaped dendrimer with a carboxyl group at the focal point with perfluorinated tails was introduced by Lee et al.38 The stability and ordering of perfluorinated, selfassembled dendrimers on water, as well as the generation of planar morphology, were investigated by x-ray diffraction (XRD), BAM, transmission electron microscopy (TEM), and AFM. In a condensed phase, perfluorinated tails are well-packed with hexagonal symmetry, and the dendrimer is self-assembled into a cylindrical mesophase. After the monolayer collapse, the interfacial and internal structure of every terrace showed planar columnar morphology. In addition, monolayer formation of two dendrimers (G1 and G2) containing a hydrophilic core group (COOH) and hydrophobic peripheral groups (anthracene and aryl ether tail groups) was studied.39 The π-A isotherm and LB monolayers show that both dendrimers form circular domains at the onset point of surface pressure as a result of the differences in hydrophobicity between the core group and the peripheral end group. The core group has a functional group at the end of dendrimer and can be anchored on the water surface. The monolayer of G1 shows a domain of molecules whereas a monolayer of G2 is aligned in the direction of compression at 10 mN/m. At higher surface pressure (20 mN/m), G1 molecules have several aggregates of domains, but G2 molecules are able to maintain their ordering.

2.3.2 Poly(amidoamine) In order to investigate the properties of hydrophilic poly(amido amine) (PAMAM) dendrimers at the air–water interface using the LB trough system, various hydrophobic functional groups at the periphery of the

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dendrimers have been modified. Sayed-Sweet et al.40 synthesized hydrophobically modified PAMAM dendrimers whose primary amino chain ends react with various epoxyalkanes (Fig. 2.10). Depending on generation, core, and surface functional groups of the PAMAM dendrimers, the Langmuir film properties on the water surface were investigated with and without copper guest molecules for developing nanoscopic container molecular systems. A disk-shaped amphiphilic dendrimer, a G4 PAMAM dendrimer core with 64 12-hydroxydodecanoic acid chains (HA-PAMAM), has been synthesized by Sui et al.41 The dendrimer with relatively small limiting molecular area (160 Å2/molecule) can form a disk-shaped structure at the air–water interface. Possible hydrogen bonding between hydroxyl groups causes edge-on configuration on the Langmuir films. Instead of the typical face-on configuration, Tanaka et al.42 and Zhang et al.43 demonstrated aggregation and molecular motion of amphiphilic hemispherical G3 PAMAM dendrimer with long alkyl chains (C12) LB films on the hydrophobic surface. The PAMAM molecules sit on the substrate with an oblate shape, in which hydrophilic core and hydrophobic alkyl end-groups are oriented towards

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the substrate and air, respectively. For further work, G0 through G5 PAMAM dendrimers with three different types of groups connecting to hydrophobic C12 tails and one type of group connecting to C18 tails were studied by Zhang et al.43 The molecular areas are significantly influenced by the size and the type of connecting group. Higher-generation (e.g. G4 and G5) amphiphilic PAMAMs with amide connecting groups are more responsive to changes in compression rate and subphase temperature and less stable than the corresponding opened epoxide- or ester-connected counterparts. Intramolecular (and possibly also intermolecular) attractive hydrogen-bond interactions between the amide connectors are proposed as the reason for this behavior. In addition, the PAMAM dendrimers incorporated with metallic particles (i.e. dendrimer nanocomposites, DNC) have been studied as self-assembled functional polymer/nanoparticle hybrid film. Seo et al.44 investigated Langmuir monolayers of amphiphilic gold/poly(amidoamine) (PAMAM) DNCs using hydrophobically modified ethylenediamine (EDA) core G2 and G4 dendrimers. The dendrimer layer is hydrated and the gold is uniformly distributed within the dendrimer body. The G2 dendrimer is spherical on the water surface, whereas the G4 dendrimer become oblate at high pressures. At high pH, the dendrimers become ‘soft’ and the films collapse at smaller areas, while at lower pH, a distinct plateau is observed which arises from crystallization of the carbon side chains. Recently, the interesting DNC Langmuir system using PAMAM dendrimer with an azacrown core was established by Ujihara et al.45 A newly designed 1.5th generation poly(amido amine) dendrimer with an azacrown core, hexylene spacers, and octyl terminals was spread on gold nanoparticle (AuNP) suspension. The surface pressure–area isotherm curves indicated that the molecular area of the dendrimer on AuNP suspension was significantly smaller than that on water, indicating the formation of dendrimer/AuNP composites. The LB films consisted of a fractal-like network of nanoparticles at low surface pressure and a defect-rich monolayer of nanoparticles at high surface pressure. The dendrimers bind AuNPs, and dendrimer/AuNP composites form networks or monolayers at the interface. The metal affinity of azacrown, flexibility of hexylene spacer, and amphiphilicity of dendrimer with octyl terminals played important roles for the formation of dendrimer/AuNP hybrid films. The present investigation proposed a new method for the fabrication of the self-assembled functional polymer/nanoparticle hybrid film.

2.3.3 Poly(propylene imine) The self-assembly of amphiphilic dendrimers based on poly(propylene imine) (PPI) dendrimers of five different generations with up to 64 end

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2.11 (a) Poly(propylene imine) (PPI) dendrimers. (b) Schematic representations of the molecular organization of the PPI dendrimers at the air–water interface.

groups modified with long hydrophobic chains has been studied by Schenning et al.46 At the air–water interface, stable monolayers form a cylindrical shape; all the chains are aligned perpendicular to the water surface, and the dendritic PPI core faces the aqueous phase. At pH 1, the amphiphiles form small spherical aggregates with diameters varying between 20 and 200 nm. The shape of the dendritic PPI core in the aggregates is distorted and the axial ratio (rb : ra) ranges from 1 : 2.5 for the first generation to approximately 1 : 8 for the three higher generation dendrimers. Su et al.47 used three amphiphilic PPI dendrimers which were synthesized by (i) attaching dodecanoyl chains to a PPI (DAB-dendr-(NH2)8) dendrimer core, (ii) reducing the amide groups to secondary amines, and (iii) methylating the secondary amines to tertiary amines as shown in Fig. 2.11. Monolayers of all three dendrimers on water are compressed to an area of 160 Å2/molecule. The amphiphilic dendrimers form stable homogeneous monolayers with surface roughness <0.5 nm on mica and a tetragonal order of a 2-D crystal with alkyl chain-to-chain spacing of 0.4–0.5 nm.

2.4

Metal/semiconductor nanoparticles

The LB technique can be adopted for characterization and applications of various nanoparticle materials, such as metals (Pt, Au, and Ag),48 magnetic materials,49 and semiconductors.50 These Langmuir films have been studied for interfacial properties and their correlation toward various potential electronic applications, such as fluorescent emitting devices, solar cells, and FET, etc.51

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Monodispersed Pt nanocrystals with three different shapes (cubes, cuboctahedra, and octahedral) with similar sizes of 9–10 nm were assembled by using the LB method.52 Surface coverage and density of the distributed particles on the entire substrate can be precisely controlled by tuning the surface pressure. It was found that the Pt LB layers with high surface coverage and structural uniformity are potential candidates for 2-D catalysts. Perez et al. have demonstrated LB films of mercaptoaniline-capped Pt nanoparticles at the air–water interface.53 The aggregates of the functionalized Pt nanoparticles are affected by external polar amine functions. The collapsed surface pressure is very low due to the polar properties of the organic shell and small size of the nanoparticles. However, mixing with an amphiphilic moiety (fatty acid) provides better film stability in which the surface pressure of a monolayer reaches as high as 30 mN/m. Despite this difference, both films exhibited good electrical stability for long periods of time. Furthermore, in order to achieve closely packed monolayers of cobalt– platinum nanoparticles over large areas by the LB technique, various experimental parameters such as dipping angle (105°), subphase type (glycol), and minimized ligands on the nanoparticle were required.54 In addition, this LB film shows narrow distribution and high ordering with temperaturedependent electrical properties. Similarly, using functionalized Au nanoparticles, various LB experiments were performed to investigate the interfacial properties at the air–water interface. Recently, Chen55 examined the electrochemical properties of thin LB films (monolayers and multilayers) of ω-ferrocenated Au nanoparticles. In the first few layers the transfer of nanoparticle onto a solid substrate surface is efficiently characterized by confirming a proportional increase of optical absorption in the UV-Vis spectrum. The interfacial dynamics of the nanoparticle monolayers via electrochemical methods revealed that the particle molecular mobility due to relatively weak interparticle interactions causes surface reorganization. As the nanoparticle thin film is closely packed, ferrocene sites are not active and accessible. With more LB layers, the films become less conductive, resulting in the anodic shift of the ferrocene formal potential. The effects of the surface pressure on the particle arrangement of LB monolayers of alkanethiol (C12)-capped AuNP were also studied.56 The LB monolayers consisting of a highly concentrated particle solution, which increases film fabrication efficiency, cause small particle voids in the particle array. At a higher surface pressure, the LB monolayer has the restructured particles and eliminates the voids. Furthermore, they constructed 2-D nanoparticle crosslinked networks by using the LB technique, where neighboring particles were chemically bridged with bifunctional linkers at the air–water interface.57 At high surface pressures, the crosslinking process was affected by ligand intercalation and surface exchange reactions between the bifunctional linkers (rigid aryl dithiols) and

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the particle-bound alkanethiolates, resulting in the formation of long-range ordered and robust particle superlattice networks. Even for particles with varied thicknesses of the protecting monolayers, effective crosslinking was achieved where the interparticle spacing appeared to be determined by the molecular length of the bifunctional linkers. One of the potential applications of the resulting particle networks is for the construction of 2-D quantum dot arrays by ultraviolet and ozone molecular cleaning, where the organic components were removed rather efficiently leaving the metal particles deposited onto the substrate surface. This could be used for efficient large-scale surface nanofabrication in a nonlithographic manner. Semiconducting quantum dots (QDs), such as CdS, CdSe, and CdTe, are of interest in various applications as well as for fundamental research.58 Recently, several studies have been carried out to investigate interfacial and physical properties of these QDs at the air–water interface using the LB technique. For example, trioctylphosphine oxide capped (CdSe)ZnS, CdSe, and CdS nanoparticles have been investigated. Trioctylphosphine oxide(TOPO-) capped (CdSe)ZnS QDs at the air–water interface was examined by the surface pressure–area isotherm.59 The Langmuir film of the QDs can form stable Langmuir films at the air–water interface, as proven by compression/decompression cycling and kinetic measurements. Gattás-Asfura et al.60 have also demonstrated Langmuir film properties of CdSe QDs in 2-D, which were stabilized by either trioctylphosphine oxide (TOPO) or 1-octadecanethiol (ODT) (Fig. 2.12). The molar absorptivity, limiting nanoparticle area, luminescence property, and arrangement of the QDs in the monolayer films at the air–water interface were determined. The TOPO forms closely packed monolayers on the surface of the

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QDs whereas ODT-stabilized QDs undergo alkyl chains interdigitation. Furthermore, cadmium sulfide (CdS) QDs modified by dodecanethiol or mercaptosuccinic acid (MSA) to render a surface with alkyl chains (C12CdS) or carboxylic acid groups (MSA–CdS) were studied.61 Owing to the hydrophobic property of C12-CdS, the nanoparticles disperse well in chloroform and stay stable at the air–water interface. However, the 3-D aggregated domains and particle-free pores are formed in the monolayer due to poor particle–water interaction. For the MSA–CdS nanoparticles, the surface is made hydrophobic through physical adsorption of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The capped MSA on the CdS plays an important role in enhancing the adsorption of CTAB and improving the stability of the QDs at the air–water interface. Due to the reversible adsorption of CTAB on MSA–CdS, a hydrophilic area can be exposed in the water-contacting region of a nanoparticle when it is at the air–water interface. Thus, it is found that the CTAB–MSA–CdS QD behaves as an amphiphilic compound at the air–water interface and has properties superior to those of C12-CdS QDs in fabrication of LBL 2-D structure of particulate films.

2.5

2-D arrays of colloidal spheres

Recently, much research has been performed on the preparation and applications of highly-organized colloidal particles at the interface. Monodisperse colloidal spheres can be self-assembled into highly ordered 2-D arrays on solid supports or in thin liquid films using a number of strategies, such as vertical lifting deposition,62 template-assisted direct self-assembly,63 electrophoretic deposition,64 convective self-assembly,65 evaporation,66 and flow cell methods.67 There are three main methodologies in forming arrays: (i) liquid–liquid interface, (ii) thin liquid film, and (iii) air–liquid interface. In the liquid– liquid interface method, Goldenberg et al.68 have demonstrated a simple and fast technique to organize a hexagonal ordered monolayer of poly(styrene–2–hydroxylethyl methacrylate) (PS–HEMA) or silica particles at the water–alkane (hexane or heptane) interfaces and transfer onto a substrate. This technique can be effectively used for applications in lithography or porous membranes. Moreover, 2-D self-assembly of crystallineordered colloids was achieved by a thin liquid film method, reported by several research groups.69 By this protocol, stable thin liquid films formed on a suitable surface can facilitate particle self-assembly through two mechanisms: convective water flow, following evaporation at the boundary of the particle arrays during self-assembly, and an organization process due to the attractive force among particles induced by the surface tension at the film surface.

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In this section, 2-D array fabrication methods at air–liquid interface, i.e. LB or LB-like process (without compressing barriers), are discussed (Fig. 2.13). The surfaces of hydrophobically modified colloidal spheres or monospheres (e.g. PS) can be partially immersed into a liquid after being spread at the air–liquid interface. A 2-D array of colloidal spheres can be effectively formed at the air–liquid interface and subsequently transferred onto the surface of a solid substrate. The surface morphology of the 2-D array changes depending on a variety of properties of the colloid particles, such as size, concentration, surface hydrophobicity, or charge density on the surface, and a composition of liquid (electrolytes or surfactants). These characteristic parameters have to be essentially considered to control attractive interactions among the colloids in self-assembling aggregates at the air–water interface. In the conventional LB method using two Teflon barriers, hydrophobicity of colloid particle surface is an important parameter. To increase the hydrophobicity, two typical methods are useful: (i) functionalization of the colloid surface; and (ii) usage of surfactants or salts. In the 1980s, Kumaki70 studied monolayer behavior and physical properties of hydrophobic monomolecular PS and silica particles with different molecular weights at the air–water interface, considering the relation between the interparticle forces and the monolayer structures. Reculusa et al.71 have reported hydrophobic silica colloidal crystals, which are synthesized via a sol–gel process and then functionalized with an appropriate coupling agent, allyltrimethoxylsilane. A well-organized 2-D/3-D silica multi-array with controlled thickness onto solid substrates was successfully fabricated by using the LB technique. 0.59 μm

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A similar method was introduced by Tsai et al.72 in which the LB method was adopted for developing the deposition of silica particles self-assembled with alkylsilane. Based on the hydrophobic-hydrophilic balance of the silica particle surface through the adsorption of surfactant molecules, deposition of monolayers consisting of hexagonally closely packed arrays of particles on a glass substrate was successfully achieved. The other method (i.e. addition of surfactants or salts) was carried out by Szekeres et al.73 The 2-D/3-D structures of monodisperse spherical silica particles (357, 450, and 550 nm in diameter) were built up with the LB technique. In the study, various ionic surfactants (hexadecyltrimethylammonium bromide, decyltrimethylamonium bromide, sodium dodecylsulfate and octylbenzenesulfonic acid sodium salt) were used to float the monolayers of silica particles on water. Furthermore, various effects of the type, concentration, and chain length of the surfactant, and the composition of the dispersion medium (chloroform, methanol, or a mixture of both) on the quality of particle ordering were investigated. Aveyard et al.74 studied the structure of monolayers of sulfate polystyrene latex particles on air–water and octane–water interfaces. If compressed sufficiently, the monolayers at air–water surfaces can have hexagonally packed particles, while those at oil–water interfaces undergo a transition from the originally hexagonal to a rhombohedral structure. Beyond the collapse, the particle monolayers on both air–water and octane–water interfaces fold and corrugate, and there is no expulsion of individual particles or particle aggregates from the interface. In the case of air–water interfaces, the structuring of particle monolayers is very sensitive to the electrolyte concentration in the aqueous phase. At low electrolyte concentration, a fairly ordered structure resulting from the interparticle repulsion is observed whereas at high electrolyte concentration, the particles form 2-D clusters. In contrast, particle monolayers at octane–water interfaces remain highly ordered as a result of long-range repulsion, regardless of the electrolyte solution. This is due to the enhanced lateral repulsion between the latex particles at the octane–water interface, attributed to the existence of residual surface charges. Several studies have been performed for developing 2-D colloidal array by a combination of the two methods mentioned above. Van Duffel et al.75 utilized ammonium-functionalized monodisperse silica spheres and sodium n-dodecylsulfate (SDS) together at the air–water surface. The resulting particulate films were deposited on various substrates and a density and refractive index of the particles were estimated from the Langmuir isotherm. The films exhibited vivid optical diffraction due to a high degree of ordering of the particles. Fulda and Tieke76 reported on the formation of relatively closely packed Langmuir films of 0.5 μm monodisperse spherical polymer particles with a hydrophobic core of polystyrene and a hydrophilic shell of poly(acrylic acid) or polyacrylamide at the air–water interface as a

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function of pH value and ion concentration in the subphase. Moreover, the influence of a non-uniform particle size on the ordering in two dimensions was studied. Lü et al.77 demonstrated a fast, flexible, simple, and highly ordered fabrication method of polymer latex spheres monolayer using the LB technique. Monolayer of polymer latex spheres, which were pretreated at 50 °C to increase hydrophobicity, was prepared at the air–water interface and deposited onto glass slides through LB technique. Hexagonally closepacked domains were found on a large scale. Thus, the hydrophilic and hydrophobic properties of the PS colloids are important factors for the formation of ordered monolayer films. Instead of the conventional LB method, several LB-like techniques without compressing by two Teflon barriers have been adopted due to their ease and simplicity for 2-D array fabrication: for example, (i) vertical or horizontal deposition, (ii) vortical surface method, and (iii) surfactantassisted method. Shimmin et al.78 have introduced a vertical colloidal layer deposition method. In the deposition of polymer latexes, where the evaporation velocity generally exceeds the sedimentation velocity, the vertically deposited colloidal crystal grows from a horizontal colloidal multilayer. This method for engineering vertically deposited colloidal crystals easily controls the thickness of the layer. Thus, the growth rate of sulfate polystyrene microspheres plays an important role in determining well-performed vertical colloidal layer deposition. However, it is difficult to achieve well-ordered 2-D arrays on large areas. In a similar way, a horizontal transfer method has been described by several research groups.79 Kondo et al.80 have utilized this method in the formation of the monolayer of alkoxylated silica particles with different chain lengths (C4, C10, and C12) at the air–liquid interface. When considering the relation between the interparticle forces and the monolayer structures, monodisperse silica particles coated with long alkoxyl chains (C12) at the air–benzene interface form 2-D polycrystalline monolayers. This is because a sufficiently weak interparticle attraction is required for the formation of ordered domains. The vortical surface method using an O-ring has been introduced by Pan et al.81 This method can rapidly fabricate a large 2-D area which is closely packed as monolayer on a vertical water surface. In the surfactant-assisted method, highly ordered hexagonal arrays of latex spheres on highly ordered pyrolytic graphite (HOPG) have been achieved from the LB-like technique using both polymers and surfactants (anionic or nonionic) as spreading agents.82 It was found that the concentration of the spreading agent in forming a well-ordered and stable monolayer at the air–liquid interface was dominant. It is only when using the anionic surfactant SDS that hexagonal arrays of latex spheres form on the surface of HOPG due to a correlation between a unique feature in surface tension measurements of the latex-spreading agent mixture and the concentrations.

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2.6

Conclusions

This chapter is an overview of surface chemical properties, surface stability, and surface morphological behavior at the air–water interface based on various organic/inorganic materials, such as polymer, dendrimers, nanoparticles, and colloids. In the past, the applicability of the Langmuir processes has been limited in academic research. However, recent studies on controlled LB films have extended their use for much wider applications both in academia and industry. In particular, techniques capable of controlling polymer morphology offer some promise for nanoscale devices. Furthermore, the method has been expanding extensively in the fields of biosensor and related nanotechnology. Therefore, the broad applicability of the technique will offer ample opportunity to develop various nanoscale analytical tools and devices, together with analysis of the nature of new amphiphilic and nanoscale materials at the air–water interface.

2.7

Acknowledgements

We would like to acknowledge the past and present members of the Advincula Research Group. Research by our group carried out in this area has received funding from: National Science Foundation (NSF) DMR-10-06776, CBET-0854979, CHE-10-41300, Texas NHARP 01846, and Robert A. Welch Foundation, E-1551. Technical support from Biolin Scientific (KSV Instruments), Agilent Technologies and Optrel is also acknowledged.

2.8

References

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3 Chemical and physical vapor deposition methods for nanocoatings I. V. SHISHKOVSKY, P. N. Lebedev, Physics Institute of the Russian Academy of Sciences, Russia

Abstract: The aim of this chapter is to provide a comprehensive analysis of vacuum deposition methods for the production of nanocoatings and functionally graded (FG) multilayers. FG layer-by-layer synthesis is generally based on a paradigm of the type connectivity of the internal structure. The chapter discusses the particularities and versatility of physical vapor deposition (PVD), chemical vapor deposition (CVD), laser-, electron- and ion-assisted technologies in the engineering of FG nanocoatings with control microstructures. The final part of the chapter describes the nanoperspectives of FG thin films and surface structures with nanoelectromechanical system (NEMS) properties and which were produced using the previously described deposition methods. Key words: Nanocoating, vacuum deposition methods, functional gradient multilayers, micro/nanoelectromechanical systems (M/NEMS).

3.1

Substrate preparation for ultra-thin films and functional graded nanocoatings

The nanocoating manufacturing process begins with the selection of an appropriate substrate. The substrate can come from the typical metallic elements (e.g., Al, Ti, Cu or Au) and alloys (e.g., Ni–Fe, Ti–Al or Pb–Sn) for industrial applications or from ceramics (e.g., AlN, GaN, TiN, SiO2, TiO2, Al2O3, ZrO2, etc.) and semiconductors (e.g., Si and quartz) for microelectromechanical (MEMS) devices such as electronic actuators, filter elements, catalytic membranes, micro pumps and drug delivery systems. The diversity of compounds used to construct nanocoatings ranges from polymer to shape memory alloys (e.g., Ni–Ti, Cu–Al–Ni), piezoelectrics (e.g., PZT), ceramics–polymers (PZT + PVDF, SiO2 + PVDF), ferromagnetics (e.g., Ba + M and Li + M systems (M ∼ Fe, Cr)) and superconductive materials (e.g., YBa2Cu3O7−x, CoFe2O4, SrFe12O19, NiCr2O4, NiFe2O4) (Shishkovsky, 2009). Owing to the small feature sizes, the nanocoating fabrication process is sensitive to surface smoothness. The surface of the substrates is therefore etched in an acidic solution to remove any remaining process damage. The etched substrates are then polished. The surface morphology of substrates can be subdivided into three main types: monocrystalline, polycrystalline, 57 © Woodhead Publishing Limited, 2011

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and amorphous. The film structure is directly dependent on the crystallographic orientation, texture, defects, crystal size, and substrate morphology. If a polycrystalline substrate is used, the orientation of the resulting film is polycrystalline too but, when using an amorphous substrate, the surface lattice will be random, although the initial deposition will favor the <111>-orientation due to the higher packing density (Jansen et al., 2004). The mechanisms are the same as in single crystal formation so that the high substrate temperature and low deposition rate lead to large grains and a low density of crystal defects. After an initial layer – covering the substrate – has formed, actual formation begins, during which the only interaction that occurs is between particles of the film material. The following parameters are very important to the results of the process: the energy of the particles arriving at the film surface, its binding energy, the absorption ability at the time of collision, the chemical and physical interaction between the adatoms and the film surface, surface mobility, deposition rate, environment pressure, and temperature. When producing a functionally graded structure it is necessary to be more particular in the choice of substrate. The choice of film material and film formation method is very important because it influences the properties of the material, such as the conductivity, structure, stress, and adhesion of the nanocoating. These properties determine the characteristics of the final product. For instance, the choice of film formation method directly affects step coverage and, depending on the film formation method used, the layer thickness can be influenced by the topography of the underlying substrate. The deposited film may also mix (alloy) with the underlying layer due to temperature-controlled diffusive mass transport, which can cause the performance of the MEMS device to degrade over time (device drifting). The film formation process should be uniform, safe to use, and cause minimal damage to structures on the substrate. Film materials can be classified into three groups: conductors (e.g., metal films), semiconductors (e.g., silicon films), and dielectric (e.g., polymers, ceramics). These films can be either amorphous (e.g., glass), polycrystalline (e.g., polysilicon), or monocrystalline (e.g., epitaxial silicon). Conducting films are used for electrical interconnection, Ohmic contact layers, rectifying metal–semiconductor contact, catalytic and seed layers, or as an electrode material in electrostatically driven devices. Semiconducting films are used as a conductive material for multilevel metallization, electronic material, or as a structural layer to create free moving MEMS structures. Dielectric layers, such as deposited silicon oxide and silicon nitride, are used between conducting layers to isolate them, for etching, diffusion, and ion implantation masks, for capping doped films to prevent loss of dopants, as a sacrificial layer in surface micromachining, and for passivation

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to protect devices from impurities, moisture, and scratches (Jansen et al., 2004). Stresses and strains that develop in interconnecting layers and structures can greatly affect their physical integrity and long-term stability. For instance, compressive or tensile stress can lead to cracking, blistering, delamination, and shape problems such as bending, shrinking, curling, or buckling of structures, as well as reliability problems such as void and hillock formation and long-term shifts in material properties. Adhesion can be improved simply by cleaning the substrate surface, but these solvents usually only remove oil and grease, leaving behind more tenacious materials such as surface oxides, which may prevent interdiffusion and thus adhesion. For gas-phase thin films, adhesion is generally promoted by including plasma pre-sputtering, which removes the surface oxide, before forming the new layer. In the case of liquid-phase film formation, chemical surface modifiers are coated prior to the new film being formed. When solid-phase-formed films are being used, surface properties such as roughness are important. It is therefore common to polish the substrate before the next layer is formed. Thin films are formed to establish specific electrical, mechanical, or chemical properties. These properties may vary significantly from those of the bulk, particularly if the film is very thin. These anomalous properties are due to the particular (nano) structure of the film which is, in turn, dictated by processes which occur during film formation. So the thin film formation method, the film thickness, and the resulting film structure greatly affect the film properties. The initially formed monolayers of a film often determine the resulting film structure, whereas the second-stage layers become apparent during the film formation. During the initial stage (nucleation, island, and coalescence) (Shishkovsky, 2010), the chemical and physical properties of the substrate and the interaction between the substrate and arriving particles (atoms or molecules) play an important role. The process of nucleation can be briefly described as follows: incoming particles collide with the substrate surface, adsorb, migrate (up to 50 nm or more), collide with each other, cluster, and finally form a stable condensate when a specific critical dimension is reached. There are several mechanisms which can cause the formation of functional oriented (gradient) layers. If the lattice constants of the substrate and the film material are an approximate match, epitaxial deposition can occur. In fact, it can often occur even when they do not match. An increase of orientation during actual deposition is caused by the different deposition rates of the crystal planes. Beginning with random crystal orientation in the initial layer, crystals with the fastest depositing crystal plane parallel to the substrate, such as the (111)-plane, ‘survive’ at the cost of the others. This

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phenomenon is also known as ‘survival of the fastest’ and implies a related increase of crystal orientation and grain size with increased film thickness (Jansen et al., 2004).

3.2

Paradigm of functional graded layer-by-layer coating fabrication

Functional graded coatings (FGCs) are a new generation of composite materials characterized by their continuously varying properties. These variations are due to continuous changes in the microstructure details, such as composition, morphology, and crystal structure, from the substrate surface up to the thin-film surface. The concept of layer-by-layer FGC is to take advantage of certain desirable features of each constituent phase, such as ceramic, metal, fiber, polymer and compounds thereof, and to optimize the distribution of material properties (strength, hardness, thermal resistance, etc.). In doing so, the desired responses to the given mechanical, thermal, electromagnetic, or biochemical loadings are achieved (Shishkovsky, 2009). FGCs can be used for various applications, including to improve the fracture toughness of machine tools, as thermal or flow gradient structures, and to increase the resistance to wear and corrosion (oxidation) of high temperature aerospace, automotive, or chemical industry components. The multilayer nanocoating approach is very beneficial to engineering applications because it can assist with the joining of structural components and microelectronics devices and would be highly suitable for use with artificial biocompatible implants. Another promising possibility for FGCs is their potential use as armour materials where the hardness of ceramics could be combined with the ductility of metals. A number of papers have been published recently concerning the vacuum web coating of acrylate films onto a variety of substrates, for a variety of applications, utilizing the polymer multilayer (PML) process for the flash evaporation of monomer fluids (Affinito, 2002). A new vacuum monomer technique (VMT), which utilizes a new low temperature source design to produce gaseous monomer, has been developed. It allows vacuum deposition of acrylate films with the same properties (ultra-smooth and pinholefree) as PML deposited films. This new VMT process should permit sub-micron or multiple-micron thick films to be deposited at web speeds in excess of 100 m/min as in the PML process (Affinito, 2002). A FG transition zone between a hard TiC coating and a WC/Co substrate, e.g. a cutting tool, can be formed over the whole composition of the titanium carbide phase which extends from Ti2C to TiC. The transition zone is formed by sputter deposition of a multilayer stack of nanometric TiC and Ti layers. The composition gradient within the carbide layer is generated by varying the relative thickness of the as-deposited Ti and TiC layers (Dahan et al.,

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2001). Plasma-sprayed functionally graded NiCoCrAlY/yttria stabilized zirconia thermal barrier coatings (TBC) could be fabricated via CO2 laser alloying (Kokini et al., 2002). A layer-by-layer (LBL) self-assembly of poly (3, 4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) on lignocellulose wood microfibres was used to make conductive fibers and paper (Agarwal et al., 2009). Polycations such as poly (allylamine hydrochloride) (PAH), and poly (ethyleneimine) (PEI) were used in alternate deposition with anionic conductive polythiophene (PEDOT–PSS) to construct the multilayer nanofilms on wood microfibres.

3.3

Nanocoating fabrication methods

After a substrate has been fabricated, the next step, generally, is to form a thin solid film (or layer) on top of the substrate. The material of this film is chosen based on its specific properties necessary for the future application or artificial micro-device to be constructed. Ultra-thin film formation generally means the deposition of a new layer on top of a substrate. Growth of a thin film, as in oxidation of a silicon substrate, differs from straight deposition due to the consumption of the substrate sub-surface material during film formation. The film can be formed on the substrate (physically or chemically) from a gas, liquid, or solid state. Gas-phase film formation methods are of key importance in microtechnology. The mechanism for physical film deposition is quite straightforward: the reactants are transported to the surface and adsorbed. Physical film growth is possible due to the subsurface implantation of high velocity particles, most often ionic species. Chemical film deposition involves three essential steps, which are identical to chemical film etching except that the direction of transport is reversed: the reactants are transported to the surface (usually via ionic species) and adsorbed. Many of aforementioned methods or improvements thereon can be considered as nanotechnology methods since they make it possible to create nanodimensional and/or nanostructural layers beyond the material surface, composite materials with nanocomponents and, in a number of cases, create the opportunity for nano- and micro-article fabrication. The approximate classification schedule for nano-oriented technology for nanocoating manufacturing is shown in Fig. 3.1 (Baloian et al., 2007). These deposition methods can be conditionally subdivided into two large technological groups, based on whether the process used is physical or chemical. Of all the nano-oriented techniques for nanocoating fabrication currently available, ionic-vacuum technology for thin film manufacturing (physical vapor deposition (PVD) and chemical vapor deposition (CVD) technology) is the most promising. We will now examine the implementation approaches in detail following Jansen et al. (2004).

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Nanocoatings and ultra-thin films Vacuum deposition methods for nanocoating Chemical vapor deposition based technologies Atmospheric pressure and low pressure CVD

Physical vapor deposition based technologies Evaporation PVD

Carbonyl pyrolysis Mex(CO)y – 2Me = yCO

Heat induction, e-beam, laser evaporation Molecular beam epitaxy

Metal-organic CVD Cathode, magnetron evaporation Plasma enhanced CVD and high density PECVD

E-beam and UVactivated CVD

Ion-plating PVD, ion implantation Laser methods Laser ablation PVD Laser alloying and amorphization

3.1 Approximate classification scheme of nanocoating technologies.

Low pressure gas-phase processes are generally preferred over the traditional chemical liquid-phase methods, such as electroplating, for the deposition of thin films used in microelectronics because deposition from an aqueous solution often produces poorer quality films. Electrochemical deposition is gaining renewed interest for use with microstructures, however, because of their emerging importance in the replication of photoresist molds. Thin films can be formed using various methods and are conveniently divided into deposition or growth out of the gas, liquid, or solid phase. The CVD and PVD methods (see Fig. 3.1) include chemical vapor growth (CVG), and physical vapor growth (PVG). The most common CVD processes are low pressure CVD, atmospheric pressure CVD, vapor phase epitaxy (VPE), plasma-enhanced CVD, metal-organic CVD, and pyrolysis. The most well known PVD processes are evaporation, laser ablation, plasma sputter deposition, molecular beam epitaxy (MBE), cluster beam, and ion plating. The vapor growth group includes only a few processes; thermal oxidation and diffusion (both CVG) and ion implantation (PVG). Liquidphase film formation methods, involving thin and thick organic materials, are of extreme importance to the construction of chemical and biological sensors. With the exception of liquid-phase epitaxy and self-assembling

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monolayers (SAMs), most chemical liquid deposition (CLD) processes are electrochemically driven, for example, electroplating, immersion plating, and electroless plating. Physical liquid deposition (PLD) processes are all varieties of coating techniques such as spinning, dipping, spraying, painting, casting, and melting. Anodization is the only practical electrochemical liquid growth (ECLG) method. Solid-phase film formation is important in packaging and the development of encapsulated mechanical structures. We can distinguish between physical solid deposition (PSD) methods, including direct, anodic, adhesive, eutectic, and compression bonding, and chemical solid growth (CSG) methods using solid-source diffusion such as doping.

3.4

Physical vapor deposition-based technologies

The basis of the PVD approach is the utilization of evaporation, transportation, and precipitation methods. The main advantages of PVD methods are as follows: the possibility of obtaining very uniform coatings, thickness from <1 nm up to 200 μm; a high reproducibility of properties; the size of the covered surface can be restricted or alternatively can be of practically unlimited length (as in the case of magnetron spraying); the possibility of selective deposition on selected sections; the almost unlimited material selection for the substrate (theoretically the wafer material can be anything); sufficient flexibility to the temperature requirements of substrate; the wide selection of coating materials (metals, alloys, chemical compounds); the possibility of obtaining multilayer and FG coating with the layers of different thickness and from different materials; the composition, structure, and properties of layers changing to account for variations in the technological parameters of deposition; the possibility of fulfilling requirements at the highest point of purity of the coating; minimal environmental pollution. The main deficiencies of the PVD approach are as follows: complexity and the high cost of technological and monitoring equipment; the necessity for very highly qualified service personnel; a comparatively low productivity; the complexity of the technological regime development for the specific case of nanocoating; the need for high precision in the chemical composition; the need for the special preparation of the coated surfaces. Unlike CVD which relies on chemical reactions to produce the reacting species to form the film, PVD methods mainly use physical processes to deposit films. PVD is more versatile than CVD, allowing for the deposition of almost any material (Fig. 3.2). PVD is performed in a vacuum either using the evaporation of a solid or molten source or by the energetic gaseous ions in a gas plasma which knock off, or sputter/dislodge, the atoms from a source target. These atoms or molecules then travel through a vacuum or a very low pressure gas phase, impinge on the substrates, and finally

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3.2 The typical installation diagram for PVD and CVD coating: 1 – coating material, 2 – system for material transfer to the vapor phase, 3 – the vapor flow, 4 – substrate, 5 – thin film or coating, 6 – system of the material transportation in the vapor phase to the wafer, 7 – focusing (and/or scanning) system of the substance flow, which is precipitated, 8 – attachment system of substrate and displacement control, 9 – temperature control system of the wafer heating, 10 – monitoring system by the technological parameters (temperature, velocity of vaporization, deposition rate, camera pressure, coating thickness, etc.), 11 – vacuum chamber, 12 – system creation and maintenance of high vacuum (system of vacuum catches, fore-vacuum and high-vacuum pumps, nitrogen trap, etc.), 13 – lock-chamber and the feed-replacement system, 14 – inspection windows, 15- cooling system, 16 – external substrate heating (laser, ion-, e-beam), 17 – CVD case delivery system.

condense on the surface to form the film. The key distinguishing attribute of a PVD reaction is that the deposition of the material onto the substrate is a line-of-sight impingement-type deposition. Sequential deposition of different films is possible if several sources are available in the chamber. Using multiple sources also facilitates coevaporation or co-sputtering to produce compounds, alloy films, or multilayer depositions. PVD reactors may use a solid, liquid, or vapor raw material in a variety of source configurations. The magnetron in some sputtering systems

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operates at voltages an order of magnitude below the e-beam source voltage and thus generates less penetrating radiation. In sputtering, the source is not heated to high temperatures and the vapor pressure of the source is not a consideration as it would be in vacuum-evaporation. Other PVD techniques, which have proven very useful in the deposition of complex compound materials such as nanotechnology, are MBE and laser ablation deposition. Ion-plating and cluster deposition are based on a combination of evaporation and plasma ionization and offer some of the advantages inherent to both techniques. Thermal evaporation PVD represents one of the oldest (∼100 years, Baloian et al., 2007) thin-film deposition techniques. Evaporation is based on the boiling off or sublimation of a heated material onto a substrate in a vacuum (positions 2–4, Fig. 3.2). The substances used most frequently for thin-film formation by evaporation are elements or simple compounds with a vapor pressure which exceeds 1 μTorr for temperatures below 2000 °C and typically deposit at a rate from 50–1000 nm/min. Three common techniques are used to evaporate materials: electron-beam (e-beam) evaporation, resistance-heated evaporation, and RF induction-heated evaporation. During electron-beam evaporation (the most popular evaporation system) a thermionic filament supplies the electron current to the beam and the electrons are accelerated by an electric field (3–20 kV) to strike the surface of the material to be deposited (e.g., aluminum) with a charge and causing it to melt locally (positions 16, 2–4, Fig. 3.2). To prevent impurities from the filament reaching the source material, it is placed in a recess in a watercooled copper hearth (position 15), a magnetic field bends the e-beam path-thus screening the impurities. Moreover, the metal forms its own crucible and the contact with the hearth is too cool for physical or chemical reactions, resulting in minimal source material contamination problems. Very high deposition rates of 1 μm/min are possible in this system, depending upon the source-to-substrate distance. E-beam heaters can achieve high temperatures so that a wide range of materials can be evaporated such as Al and its alloys, Si, Ti, Mo, W, Pd, Pt, etc. and several dielectrics such as SiO2. One disadvantage of the e-beam process is the generation of X-rays by the e-beam. This ionizing radiation can penetrate the surface layers of the devices, causing damage such as the creation of oxide-trapped charges. Subsequent annealing is therefore required to remove such damage. The x-ray damage can be avoided by using a focused, high power laser beam (see section on laser ablation). EB–PVD technology is being explored as a means of forming net-shaped components for many applications, including the space, turbine, optical, biomedical, and auto industries. Coatings are often applied to components to extend their performance and life under severe environmental conditions, including thermal, corrosion, wear, and oxidation. In addition,

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coatings have been used in designing and developing sensors. The performance and properties of the coatings depend upon their composition, microstructure, and deposition condition. Singh and Wolfe (2005) demonstrated FGCs and nano-laminated coatings using this technique with different metallic and ceramic coatings including chromium, titanium carbide (TiC), hafnium carbide (HfC), tantalum carbide (TaC), hafnium nitride (HfN), titanium-boron-carbonitride (TiBCN), partially yttria stabilized zirconia (YSZ), and HfO2-based TBC coatings. When using a resistance-heated source, a refractory metal (i.e., a metal such as tungsten with a high melting point) is coiled into a filament or formed into a containment structure and a small piece of the material to be deposited (e.g., aluminum) is placed inside. Resistance-heated evaporation is simple and inexpensive and produces no ionizing radiation. Its disadvantages include the possible contamination from the heater filament and a limited film thickness because of the small charge in comparison to beam evaporation. Furthermore, resistance heaters cannot achieve temperatures as high as those of e-beam heaters so that a smaller range of materials can be evaporated. For example, refractory metals such as platinum, molybdenum, tantalum, and tungsten do not easily heat to the temperatures required to reach a sufficient vapor pressure. Laser ablation PVD uses intense laser radiation to erode a target (position 16, Fig. 3.2) and deposit the eroded material onto the substrate. A high energy focused laser beam avoids the x-ray damage to the substrate encountered with e-beam evaporation. A high-energy excimer laser pulse coming from, for example, a KrF laser at 248 nm with pulse energy in the focus of 2 J/cm2 is directed onto the material to be deposited. The energy of the very short wavelength radiation is absorbed in the upper surface of the target, resulting in an extreme temperature flash, which evaporates a small amount of material. This material, partially ionized in the laser-induced plasma, is deposited onto a substrate almost without decomposition. This technique is particularly useful when dealing with complex compounds, as in the case of the deposition of high temperature superconductor films, for example, Yba2Cu3O7−x. Pederson et al. (2006) showed that laser ablation allows thin, fully dense, and oriented air electrode films to be deposited onto various electrolytes. A matrix-assisted laser desorption/ionization (MALDI) target plate is coated with an omniphobic polysilazane nanocoating and has an array of parallel grooves acting as recipients to the separation effluent. The threedimensional (3D) pattern in the top layer of the coated plate greatly improves loading of the matrix solution prior to separation and facilitates deposition of the separated species. Amantonico et al. (2009) and Sellinger et al. (2006) demonstrated poly (methyl methacrylate) (PMMA) film preparation using matrix-assisted pulsed-laser evaporation (MAPLE).

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Sputtering PVD is preferable to evaporation in many applications due to the wider choice of materials to work with, better step coverage, and better adhesion to the substrate. Several sputter systems exist, such as dc, reactive, RF, and magnetron sputtering. During the sputter technique, an inert gas is fed into the reactor at low pressure. A voltage is applied across two electrodes and plasma is created (positions 2,4, Fig. 3.2). The bottom electrode, the cathode or ‘target’ where a negative dc voltage is applied, is actually the source material to be deposited, a plate of aluminum for example. The top electrode, the anode where the substrates are located, is another metal plate and is grounded. The positive ions from the plasma source, usually Ar+, are accelerated through the potential gradient where they bombard the target. Through momentum transfer, atoms near the surface of the target material become volatile and are transported as a vapor to the substrate. Once the vapor reaches the substrate, the film forms through deposition (the vapor condenses). Since the target acts as an electrode in the dc mode of sputter deposition, the target source material must be conductive. Therefore, Al, W, Ti, silicides, and other metallic components can be sputtered this way. Sometimes sputtering of the substrate is desirable, for example, to improve adhesion. In such cases, a negative bias is applied to the substrate electrode. Positive argon ions from the plasma can then be accelerated to the substrate and sputter off atoms (positions 6, 7, Fig. 3.2), thus removing any contaminants or native oxides. In addition, adhesion is improved due to the creation of surface dangling bonds by the ion impact. Nanostructured ceramic coatings produced by plasma spray process are being developed for a wide variety of applications that require resistance to wear, erosion, cracking, and spallation. During reactive sputtering, a reactive gas is introduced in the reactor in addition to the argon plasma, and the compound is formed by the elements of that gas combining with the sputtered material. For example, TiN can be deposited by sputtering Ti in the presence of nitrogen. DC sputtering is not suitable for insulator deposition. Chaiwong et al. (2007) showed that TiN films were deposited on a polycarbonate substrate by a cathodic vacuum arc using plasma immersion ion implantation. Titanium dioxide (TiO2) thin films are widely used in various applications, such as corrosion protection, solar cells, gas sensors, self-cleaning surfaces, photocatalytic surfaces, and capacitors because of their unique dielectric, photocatalytic, and transparent ferromagnetic material properties and their high chemical stability. Recently, sputtering technologies such as magnetron sputtering have been used to apply TiO2 coatings to polymer substrates (Chun et al., 2008). Molecular-beam epitaxy PVD is an epitaxial process involving a reaction under ultra-high vacuum conditions (<10−10 Torr). In MBE, a heated single

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crystal (400–800 °C) is placed in the path of streams of atoms from heated cells that contain the material to be deposited. These atomic streams impinge, in a line-of-sight fashion, on the surface-creating layers with a structure controlled by the structure of the crystal surface, the thermodynamics of the constituents, and the sample temperature. MBE can achieve precise control in both chemical compositions and doping profiles. Singlecrystal multilayer structures with the dimensions of atomic layers can be created using MBE. The deposition rate of MBE is very low (about 20 nm/ min), which accounts for the ultra-high vacuum needed to obtain high purity epilayers. Because MBE uses an evaporation method, the basic kinetic theory of gases in a vacuum system has to be considered. During cluster-beam PVD the ionized atom clusters (100–1000 atoms) are deposited on a substrate in a high vacuum (10−5–10−7 Torr). Those atom clusters typically carry one elementary charge per cluster and therefore achieve the same energy in an electrical field as a single ion would. To create these atom clusters, a special evaporation cell must be used. Evaporant heating in an evaporation cell with a small opening causes adiabatic expansion, from more than 100 to 10−7 mbar, of the vapor upon exiting that cell. The expansion causes a sudden cooling, inducing the formation of atom clusters. These clusters are then partially ionized by an electron bombardment from a heated filament. Low energy neutral clusters (0.1 eV) and somewhat higher energy ionized clusters (a few eV) arrive at the surface where they flatten and form a film of excellent adhesion and purity, with a relatively low number of defects. Cluster-beam epitaxy is possible at temperatures as low as 250 °C, and no charge buildup occurs when depositing on an insulator. During ion-plating PVD, the evaporation of a material is combined with ionization (position 16, Fig. 3.2) of the atom flux using an electron filament or plasma. The addition of a gas (e.g., nitrogen) to the reactor enables new compounds, such as TiN, to be created on the substrate surface. The gas reacts with the ionizing atoms from the evaporation source (Ti) and, because of the high kinetic energy of the impacting ions, a very well adhering, dense TiN film with extraordinarily low friction coefficient and high hardness coefficient (Vickers hardness of 50 000) forms. The thermal nature of the process means that very high deposition rates can be achieved. During the ion implantation process, the dopant ions (e.g., B+ and As+) are implanted into the semiconductor by means of a high energy ion beam. From the window (position 16, Fig. 3.2), ions transfer through the management system (positions 6,7, Fig. 3.2) to substrate 5. Typical ion energies are between 30 and 300 keV, and typical ion doses vary from 1011 to 1016 ions/ cm2. The high energy ion beam passes through vertical and horizontal scanners and is implanted into the semiconductor substrate. The energetic ions lose their energy through collisions with electrons and nuclei in the

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substrate and finally come to rest. Owing to the kinetic energy of the impinging ions, the doping concentration peaks inside the semiconductor and the profile of the dopant distribution is determined mainly by the ion mass and the implanted-ion energy. The advantages of the ion implantation process are: precise control of the total amount of dopants, improved reproducibility of impurity levels, and lower temperature processing. For heavy ions, the energy loss is primarily due to nuclear collisions; therefore substantial lattice disorder (damage) occurs. When the displaced atoms per unit volume approach the atomic density of the semiconductor, the material becomes amorphous. For 100 keV arsenic ions, the dose required to make amorphous silicon is 6 × 1013 1/cm2. Much of the energy loss for light ions (e.g., B+ in silicon) is due to electronic collisions, which do not cause lattice damage. The ions lose their energy as they penetrate deeper into the substrate. Eventually, the ion energy is reduced sufficiently for nuclear stopping to become dominant. Most of the lattice disorder, therefore, occurs near the final ion position at a depth of up to a few hundred nm. Surface polymerization by ion-assisted deposition (SPIAD) allows conducting polymer thin films to be grown on substrates by the simultaneous deposition of hyperthermal polyatomic ions and thermal neutrals in vacuum (Hsu et al., 2007). The polymer-assisted deposition (PAD) method addresses some of the limitations of sol–gel and costs of high vacuum techniques. PAD utilizes an aqueous polymer to bind a metal or metal complex which serves both to encapsulate the metal to prevent a chemical reaction and to maintain an even distribution of the metal in solution. Another advantage of PAD is that the same solution can be used as a precursor for the growth of metal oxide or reduced metal films (Shukla et al., 2006). A specific method of deposition is supersonic free-jet PVD (SFJ-PVD) for coating. This technique was used to deposit nanoparticles with supersonic gas flow and to form a thick coating film without a crack or a void. This method is composed of ‘gas evaporation’ and ‘vacuum deposition’. In a gas evaporation chamber, a source material is evaporated to form nanoparticles in an inert gas atmosphere. The nanoparticles are then carried to a substrate in a deposition chamber with an inert gas flow through a transfer pipe (Niwa et al., 2007). SFJ-PVD was used for FGC fabrication from an Al/AlTi and Al/Al-Si nanocomposite coating onto an A1050 Al alloy substrate (Yumoto et al., 2004). FG coatings could be fabricated via the cold and detonation spray methods (Fig. 3.3) for medical applications. High strength, ductility and absence of hydroxyapatite destruction (Fig. 3.3c) are the confirmed results of the energy dispersive x-ray analysis used to characterize such coatings. Primary ceramic nanoparticles or metal insulating varistors could be conformally coated with nanothick alumina films using atomic layer

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3.3 Cross-section view of results of D-gun spay deposition of functional gradient coating from titanium and hydroxyapatite on the stainless steel substrate. Optical metallography (a) and SEM image (b) with dates of EDX analysis (c). Around (c) clockwise the place and element content are shown: phosphorus, oxygen, carbon, calcium, silicon, and titanium. (Unpublished results of Shishkovsky at ENISE (DIPI laboratory, France)

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deposition (ALD) (Hakim et al., 2005; Waimer et al., 2005). Titania, silica, and zirconia nanoparticles were uniformly coated in a fluidized-bed reactor at low pressure and under mechanical vibration. This process yields nanothick films that are conformal, non-granular, and pinhole free. The selflimiting, self-terminating characteristics of ALD allow for a precise control over the film thickness.

3.5

Chemical vapor deposition-based technologies

During CVD, the constituents of a vapor phase, often diluted with an inert carrier gas, are introduced into a reaction chamber (position 17, Fig. 3.2) and adsorbed on a heated surface. The atoms chemically react and migrate before forming the desired film. In CVD, the diffusive–convective transport to the substrate involves many intermolecular collisions. The reactions which form the solid material do not, therefore, always occur on or close to the heated substrate (heterogeneous reactions), but can also occur in the gas phase (homogeneous reactions). Homogeneous films have poor adhesion, low density, and a high defect rate (pinholes) so heterogeneous reactions are preferrerable. The most favorable end product of a CVD reaction is a stoichiometric-correct film. Several activation barriers need to be surmounted to arrive at this stoichiometric film. An energy source, such as thermal, photons, electron, or ion bombardment, is required to achieve this. The CVD method is very versatile and can create amorphous, polycrystalline, uniaxially-oriented polycrystalline and epitaxial layers with a high degree of purity and control. Figure 3.2 illustrates the common scheme and reaction processes underlying CVD with the numbers representing the following: 1. mass transport of reactant gases and diluents in the bulk gas flow region to the deposition zone; 2. homogeneous gas-phase reactions leading to film precursors and by-products; 3. mass transport of film precursors and reactants to the substrate surface; 4. adsorption of film precursors and reactants on the substrate surface; 5. heterogeneous surface reactions of adatoms occurring selectively on the heated surface; 6. surface migration of film formers to the deposit sites; 7. incorporation of film constituents into the depositing film, that is, nucleation (island formation); 8. desorption of by-products of the surface reactions; 9. mass transport of by-products in the bulk gas flow region away from the deposition zone (Jansen et al., 2004). It is, however, often sufficient to consider the fluxes for mass transport (i.e., diffusion) and surface reaction processes only and equate them with steady-state conditions: diffusion-controlled, thermally-activated, and diffusional-thermionic processes. The mass transport in the gas phase takes place through diffusion proportional to the diffusitivity of the gas – D and the concentration gradient – C across the boundary layer that separates the

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bulk flow (source) and substrate surface (sink). The surface-reaction processes can be modeled by a thermally-activated phenomenon. In general, the deposition rate will depend on both the mass-transport and the surfacereaction processes. At high flow rates, the growth rate reaches a maximum and then becomes independent of flow. In this regime, the surface-reaction rate controls the deposition, regardless of the flow rate. This regime is referred to as surface-reaction limited deposition. At high temperatures, however, the reaction rate cannot proceed any faster than the rate at which the reactant gases are supplied to the substrate by mass transport, no matter how high the temperature is raised. The most common CVD processes are atmospheric pressure CVD (APCVD) and low pressure CVD (LPCVD), VPE, plasma-enhanced CVD (PECVD), high density PECVD (HDPECVD) and pyrolysis. For nanocoating fabrication both very low pressure CVD (VLPCVD) and metalorganic CVD (MOCVD), which uses metal-organic compounds to grow films, could prove to be interesting approaches. The thermal energy is the sole driving force in high temperature CVD reactors such as APCVD and LPCVD. In a hot-wall LPCVD reactor, a quartz tube is heated by a three-zone furnace and gas is introduced at one end and pumped out at the opposite end. The quartz tube wall is hot because it is adjacent to the furnace. Typical reaction chamber pressures vary from 30–250 Pa (0.25–2 Torr) and temperatures are normally 300–900 °C. Among the benefits of this reactor is the fact that it deposits films with excellent uniformity and in large throughput. The gases used, however, may be toxic, corrosive, or flammable. The APCVD reactor is similar to a resistance heated hot-wall oxidation furnace except that different gases are used at the gas inlet. In contrast, in a cold-wall APCVD reactor, such as the epitaxial reactor, the substrates are heated using a graphite susceptor, which is heated by RF induction. A direct practical application of the possible rate-limiting processes is to compare the way in which substrates are stacked in LPCVD and APCVD reactors. In a LPCVD reactor, the diffusion of the gas species is increased by a factor of 1000 over that in APCVD due to the difference in pressure, resulting in a one order of magnitude increase in the transport of reactants to the substrate. The rate-limiting step becomes the surface reaction. Hence, it is easier to achieve a uniform temperature distribution than a uniform arrival of gas species across a substrate surface. Deposition is, therefore, more uniform at higher flow and lower temperature settings. LPCVD reactors enable substrates to be stacked vertically with very close spacing as the rate of arrivals of reactants is less important. Polysilicon is usually deposited at lower temperatures in LPCVD systems and in the surface-reaction regime. Here the gas-phase processes are not important so the stacking of substrates is possible, but the deposition is very temperature sensitive.

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APCVD, however, operating in the mass-transport-limited regime, must be designed so that all locations on the substrate and all substrates are supplied with an equal flux of reactant species. In this case, the substrates are often placed horizontally. The VPE process is the most promising of the various epitaxial methods. Epitaxial Si deposition has traditionally been performed at higher temperatures (typically in APCVD systems) to ensure that all the Si atoms being deposited are incorporated into lattice sites in order to obtain a singlecrystal thin film. This means that epitaxial Si in these systems is deposited in the mass-transport regime where gas-phase processes are important. The epitaxial process offers an important means of controlling the doping profiles so that device and circuit performance can be optimized. Very low-temperature pyrolysis LPCVD will become more important as micro-devices begin to incorporate materials (e.g., aluminum or polymers) not able to withstand high post-temperatures. To improve step coverage, sometimes metals can be deposited by MOCVD using metal-organic compounds. Sometimes there are restrictions on the temperature that the substrate can be exposed to when depositing a film. For lower temperature deposition, an additional energy source is needed, such as that found in PECVD. PECVD is an energy-enhanced CVD method in which plasma energy has been added to the thermal energy of a conventional CVD system. The reactions and processes that occur in a PECVD system are complicated and difficult to predict. The complexity of the plasma reactions can lead to nonstoichiometric compositions of films (e.g., Si-rich SiO2), as well as the incorporation of H2 or N2 by-products into the film. This can result in out gassing, peeling, or cracking of the film during subsequent processing. Film density and stress may also vary, depending on the conditions of the deposition. PECVD can result in a fairly good coverage and filling of non-linear topography, better than what might be expected at these low temperatures, probably due to surface emission and redeposition events caused by ion bombardment. HDPECVD utilizes very high density plasma and a separate RF bias applied on the substrate to obtain a very good filling of narrow gaps. Coating of powders is often used to protect particles and to modify surface properties such as lustre, catalytic activity, hardness, permeability, adhesion, and conductivity. A fast circulating, fluidized bed for combustion chemical vapor deposition (CCVD) and CVD is introduced for the production of particles coated with nanolayers of metal oxides and metals (Oliaca et al., 2005; Shishkovsky, 2009). The CCVD fluidized-bed process involves Nanomiser® device atomization, the controlled thermal decomposition of low cost metal-organic liquid precursors, and the nanocoating of particles with thin films or with nanoparticulates while suspended in a turbulent fluidized bed.

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The CVD (PECVD) of polymers combines the organic synthesis of polymers, typically performed in the liquid phase, with the formation of coatings from the vapor phase, widely used in the formation of inorganic thin films. CVD polymers integrate directly into vacuum processing schemes for the fabrication of inorganic optoelectronic devices, which demand high purity films and interfaces. Purity is also paramount for biomedical device coatings, where residual solvent rather than the polymer coating itself can be responsible for a lack of biocompatibility and poor electrical insulation characteristics. CVD polymerization has also been used in the manufacture of micrometer-scale resistive sensors (Asatekin et al., 2010). Since it uses only a low energy input to drive selective chemistry, modest vacuum, and roomtemperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. Most of the low-temperature methods use energy-enhanced CVD techniques (e.g., a focused e-beam) to deposit dielectric films between 25 and 300 °C. One energy-enhanced CVD method uses UV radiation to form vapor phase reactants that enhance the deposition rates. Silicon dioxide films have been deposited at a rate of 15 nm/min at temperature as low as 50 °C by means of UV radiation.

3.6

Conclusion and future trends

Developing near-net shape methods of nanocoating manufacturing is of vital importance for the future of nanotechnology. Current opportunities for nanomaterials lie in the fabrication of nanostructural FGCs with very hard cutting tools, the superplasticity of ceramics during processing, high performance parts for the aerospace and construction industries, energy and filter technologies (novel solar cells and water purification), the automobile industry, optical and catalytic applications, and sensors. In the longer term, artificial MEMS devices will find a substantial market in the electronic industry with applications in (opto) electronics and photonics. Interdisciplinary cooperation in research and development is important for the realization of scientific breakthroughs and for new products such as hybrid nanocoatings or nanoelectronic devices. In order to form a nanofilm accurately, it is important to have a process in which it is possible to deposit or grow the film in such a way that the thickness, composition, and structure are guaranteed within certain specifications. Consequently, when forming nanofilms, film formation processes which are sensitive to particle creation (such as homogeneous gas-phase reaction in CVD or dusty plasmas) and processes with high film-formation speed are best avoided as are particles. Typical processes which are able to form nanolayers are LPCVD, MBE, laser ablation deposition, thermal and chemical oxidation, anodization, spin coating, and SAMs. These nanomate-

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rials exhibit new or improved properties compared to the corresponding bulk material. This makes them attractive for use in applications such as quantum dot lasers, (electro) luminescent devices, novel solar cells, gas sensors, resistors and varistors, conductive and capacitive films, high temperature superconductors, and thermoelectric, optical, and magnetic films. Examples of nanostructured films are nanoporous, nanocrystalline, nanocomposite, and hybrid films. Nanoporous films have nanosized pores; nanocrystalline material consists of many crystalline domains; and nanocomposite film contains two or more phase-separated components with a morphology of spheres, cylinders, or networks with nanodimensions. Nanohybrid films are constructed from a combination of polymeric organic components and inorganic or ceramic components which are chemically interconnected on a molecular level. Both the size of the nanostructure and the scale of order within them affect the film properties, and nanostructured films often exhibit properties that are drastically different from those of conventional films. In many cases, this is a result of the large fraction of grain boundaries (the boundaries between the nanoparticles in the film) and consequent increase in the number of surface atoms. This surface can border on the embedding matrix, on a nanoparticle, on air, or on a vacuum in case of a pore or defect. For this reason, film properties become governed by surface properties. The size range of 1–100 nm implies a number of atoms per particle varying from several to ten million or more. For the smallest sizes, the surface-to-volume ratio becomes very large. Nanoscale multilayer coatings, which consist of alternating layers of materials, further improve the performance of single-layer nanostructured coatings. The types of materials, their bonding characteristics, and crystal structures differentiate multilayer coatings. When properly tailored, nanomultilayer coatings produce superhardness and supermodulus effects. The significant potential of nanostructured material combinations is virtually unexplored. There is a great possibility that combinations can be found which exhibit superhardness while also possessing other excellent wear properties, such as high temperature hardness, fracture toughness, chemical inertness, and a low coefficient of friction. The superior mechanical and physical properties of nanostructured coatings make them well suited for extreme operating conditions such as aerospace applications. Nanostructured films are being increasingly used in MEMs devices due to their superior friction and wear resistance, which are the major limitations for MEMs.

3.7

References

Affinito J (2002), ‘A new method for vacuum deposition of polymer films’, Thin Solid Films, 420–421, 1–7.

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Agarwal M, Lvov Y and Varahramyan K (2006), ‘Conductive wood microfibres for smart paper through layer-by-layer nanocoating’, Nanotechnology, 17(21), 5319–5325. Amantonico A, Urban P L and Zenobi R (2009), ‘Facile analysis of metabolites by capillary electrophoresis coupled to matrix-assisted laser desorption/ionization mass spectrometry using target plates with polysilazane nanocoating and grooves’, Analyst, 134(8), 1536–1540. Asatekin A, Barr MC, Baxamusa S H, et al. (2010), ‘Designing polymer surfaces via vapor deposition’, Material Today, 13(5), 28–36. Baloian B M, Kolmakov A G, Alumov M I and Krotov A M (2007), Nanomaterials. Classification, Properties, Applications and Methods of Fabrication, Moscow; Dubna University. Chaiwong C, McKenzie D R and Bilek M M (2007), ‘Study of adhesion of TiN grown on a polymer substrate’, Surface and Coatings Technology, 201, 6742–6744. Chun D M, Kim M H, Lee J C and Ahn S H (2008), ‘TiO2 coating on metal and polymer substrates by nano-particle deposition system (NPDS)’, CIRP Annals – Manufacturing Technology, 57, 551–554. Dahan I, Admon U, Frage N, Sariel J, Dariel M P and Moore J J (2001), ‘The development of a functionally graded TiC/Ti multilayer hard coating’, Surface and Coatings Technology, 137, 111–115. Hakim L F, Zhan G, George S M and Weimer A W (2005), ‘Conformal coating of ceramic nanoparticles via atomic layer deposition’, AIChE Annual Meeting, Conference Proceedings, 4507. Hsu W-D, Tepavcevic S, Hanley L and Sinnott S B (2007), ‘Mechanistic studies of surface polymerization by ion-assisted deposition’, Journal of Physical Chemistry C, 111(11), 4199–4208. Jansen H V, Tas N R and Berenschot J W (2004), ‘MEMS-based nanotechnology’, In Nalwa H S (ed.), Encyclopedia of Nanoscience and Nanotechnology, Volume 5, Number 1, Stevenson Ranch: CA, American Scientific Publishers, 163–277. Kokini K, DeJonge J, Rangaraj S and Beardsley B (2002), ‘Thermal shock of functionally graded thermal barrier coatings with similar thermal resistance’, Surface and Coatings Technology, 154, 223–231. Niwa N, Yumoto A, Yamamoto T, Hiroki F and Shiota I (2007), ‘Coating with supersonic free-jet PVD’, Materials Science Forum, 561–565, 981–984. Oljaca M, Sundell S, Smith P, Hunt A and Tellefsen M (2005), ‘Surface coating of particles by nanospray process and CCVD in circulating fluidised bed’, Surface Engineering, 21(1), 47–52. Pederson L R, Singh P and Zhou X-D (2006), ‘Application of vacuum deposition methods to solid oxide fuel cells’, Vacuum, 80, 1066–1083. Sellinger A T, Leveugle E M, Gogick K, Zhigilei L V and Fitz-Gerald J M (2006), ‘Laser processing of polymer nanocomposite thin films’, Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 24(4), 1618–1622. Shishkovsky I V (2009), Laser Synthesis of Functional Mesostructures and 3D Parts. Moscow: Fizmatlit. Shishkovsky I V (2010), ‘High speed laser assisted surface modification’, In Hamdy A S (ed.), High Performance Coatings for Automotive and Aerospace Industries, New York: Nova Science Publishers, 109–126.

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Singh J and Wolfe D E (2005), ‘Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB–PVD)’, Journal of Materials Science, 40(1), 1–26. Shukla P, Lin Y, Minogue E M, Burrell A K, McCleskey P M, Jia Q and Lu P (2006), ‘Polymer assisted deposition (PAD) of thin metal films: A new technique to the preparation of metal oxides and reduced metal films’, Materials Research Society Symposium Proceedings, 893, 183–188. Weimer M A., King D M, Hakim L F, Zhan G, Weimer A W (2005), ‘Fabrication and electrical characterization of ultrafast transient surge suppression devices based on aid surface modified varistor materials’, AIChE Annual Meeting, Conference Proceedings, 3054. Yumoto A, Yamamoto T, Hiroki F, Shiota I and Niwa N (2004), ‘Al/Al-Si nanocomposite graded coating prepared by supersonic free-jet PVD’, Materials Transactions, 45(8), 2740–2743.

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4 Surface-initiated polymerisation for nanocoatings V. HARABAGIU, L. SACARESCU, A. FARCAS, M. PINTEALA, and M. BUTNARU, ‘Petru Poni’ Institute of Macromolecular Chemistry, Romania

Abstract: Thin polymer layer–surface conjugates are appropriate materials for the study of surface/interface physico-chemical properties and of their interactions with the environment. They allow control over the performance of the entire system. It is in relation to these factors that the chapter presents recent advances in surface-attached polymer layers. The thermodynamic and kinetic aspects of polymer physical and chemical sorption are discussed first. The second part summarises the preparatory methods for polymer-grafted surfaces with an emphasis on controlled processes which are capable of producing well-defined polymer–surface objects. Finally, the unique properties of polymer brushes, when compared to bulk characteristics or to physically adsorbed layers, are highlighted in connection with specific applications. Key words: polymer-grafted surfaces, physisorption, chemisorption, equilibrium and non-equilibrium adsorption, grafting to, grafting from, grafting through, speciality materials.

4.1

Introduction

The behaviour of materials is interdependently controlled by both their bulk and surface properties. Most of the materials encountered in daily life are those which are used in a variety of high-tech applications as well as those comprising living organisms which are not pure compounds but are formed by mixtures of phases kept together by specific interfaces. These interfaces have a strong influence on the bulk properties of the whole material, while the surface and interfacial characteristics are determined by the bulk chemical structure and composition. (Jones and Richards, 1999). That is why surface modification, which is designed to provide specific properties and interactions, became a widely researched topic driven by the availability of advanced methods of surface investigation at the atomic, molecular and nano levels. Polymer chains were physically or chemically attached to inorganic (metal, oxides, salts, etc.) or to organic (polymers) flat, curved or porous surfaces in order to tailor properties such as wetability, resistance to environmental or technological physico-chemical agents, adhesion, biocompatibility, lubricity, etc. 78 © Woodhead Publishing Limited, 2011

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There is a huge range of applications from natural surfaces (Fuchs, 2007; Koch et al., 2009; Yanik, 2009; Zhang et al., 2009], through various processes such as catalysis (Ertl, 2008), filtration (Wegmann et al., 2008), chromatography (Bruch et al., 2009), to industrial products such as paints (Matheson, 2006), adhesives (Small and Courtney, 2005) or lubricants (Raviv et al., 2003). In all instances, the properties and structure of the molecules close to the interface are significant when compared to the bulk properties (Butt and Graf, 2003). Controlling the structure/microstructure and dimensions of the interface layer is also of great importance in molecular electronics (Akkerman et al., 2006), microelectronics, photonics (Chen et al., 2005), microelectromechanical systems (MEMS) (Craighead, 2000) or sensor technology (Bretagnol et al., 2006) and biology (Agheli et al., 2006). While various techniques have been developed for the preparation of structured self-assembled monolayers (Woodson and Liu, 2007), only a few studies have been devoted to micro- and nanostructured polymer grafts (Wegmann et al., 2008). New strategies need to be developed for the preparation and characterisation of these new nanomaterials. Sensor technologies (Bailey and Hupp, 2003), combinatorial science (Stoykovich et al., 2003), biomedicine (Nath and Chilkoti, 2002) and nanofluidics (Beebe et al., 2000) are only a few examples of fundamental and applied research areas in which the preparation of structured polymer layers having control over chemical functionality, shape and feature dimensions on the nanometer scale, may open up new perspectives.

4.2

Physisorption and chemisorption, equilibrium and irreversible adsorption

The adsorption of macromolecular chains to different surfaces or interfaces has promoted intensive theoretical and experimental studies designed to advance understanding of the mechanisms of adsorption and the properties of the adsorbed layer. The adsorption of small and large molecules to interfaces is a fundamental phenomenon which controls many natural (Fleer et al., 1993) and technological processes. The attachment of polymer layers with significantly higher densities than those of bulk polymer concentrations was found by contacting surfaces with evenly diluted polymer solutions (Bouchaud and Daoud, 1987).

4.2.1 Thermodynamics and kinetics The adsorption process is described by both equilibrium and nonequilibrium processes.

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Equilibrium adsorption It is more than two decades since the equilibrium properties of adsorbed polymers were clarified (de Gennes, 1987; de Gennes, 1989; Lee et al., 1991). The formation of equilibrium layers was further theoretically supported (Semenov and Joanny, 1995; Aubouy et al., 1996; Semenov et al., 1996) and studied by numerical simulation (Lai, 1995; Zajac and Chakrabarti, 1996; de Joanny et al., 2001). It was clearly stated that the interface polymer layer is composed of segments bound to the surface as well as of loops and tails, which extend the layer in the bulk phase. Typical examples of equilibrium adsorption are the physisorption processes, which involve weak interactions of the order of kT between each monomer and the surface (weak physisorption). Irreversible adsorption Evidence of non-equilibrium polymer adsorption dynamics has been found (Johnson and Granick, 1992; Minko et al., 2000), based on advanced experimental techniques, such as atomic force microscopy (Milling and Kendall, 2000; Yamamoto et al., 2000; Roiter and Minko, 2005, 2007; MierczynskaVasilev and Beattie, 2010), which allows the manipulation of a single polymer chain on surfaces, or neutron reflectometry (Marzolin et al., 2001). Additionally, computer simulations supported, or even preceded, the experimental findings, thus providing a valuable insight into the static and dynamic phenomena at interfaces (Eisenriegler, 1993; Fleer et al., 1993; Shaffer and Chakraborty, 1993; Raviv et al., 2002; O’Shaughnessy and Vavylonis, 2003a; O’Shaughnessy and Vavylonis, 2005). Both strong physisorbed and chemisorbed systems showed irreversible features, but with very different kinetics (O’Shaughnessy and Vavylonis, 2003a; Panja et al., 2009). For strong physisorption, the kinetics is diffusioncontrolled in the absence of a significant free energy barrier, while in chemisorption, the existence of a large activation energy barrier slows down the process by more than eight orders of magnitude (O’Shaughnessy and Vavylonis, 2003a). Other parameters influencing the dynamics of polymer adsorption are the bulk concentration of the polymer (dilute or concentrated solution and melts), solvent nature, polymer functionality and polydispersity (Minko, 2008). For polymer irreversible adsorption from a diluted solution, considering the reaction rate k(s) as a function of the sticking rate (Q) and a θ power of the loop length (s) (Eq. 4.1), O’Shaughnessy and Vavylonis (2003a) have established three possible mechanisms as a function of a critical reaction exponent, θ, and solvent nature (relations 4.2–4.4). k(s) = Q/sθ

[4.1]

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Chain collapse: many monomers on the polymer chain can interact at once with the surface: Qτads ≈ constant for θ < 1

•

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[4.2]

Zipping mechanism: once the first monomer is attached to the surface, its immediate neighbour will attach in a continuing process: Qτads ≈ N θ−1 for 1 < θ < 2

[4.3]

θ being 8/5 and 3/2 in good and theta solvents, respectively. • Accelerated zipping: Qτads ≈ N for θ > 2

[4.4]

where τads is the adsorption time and N is the number of monomer units in the polymer chain. Penn et al. (2004) studied the chemisorption kinetics of amino-functionalised polystyrene chains on epoxy-functionalised flat surfaces in the presence of good and poor solvents. Figure 4.1 shows the comparative kinetic curves for amino mono-functional polystyrene chemisorption, as derived from their reported results. The system characteristics correspond to a true surface-linked polymer brush (2Rg > d; Alexander, 1977; Fleer et al., 1993; Szleifer and Carignano, 1996), since the radius of gyration of polystyrene, Rg, was about 2.07 nm and the average distance between two adjacent anchoring points on the surface was 3.9 nm.

Surface tethering density (sq mn)

0.12 (b)

0.1 0.08

O CH CH2 + H2N PSt

OH CH CH2 NH PSt

(a)

0.06 0.04 0.02 0

1

10

100 1000 Time (min)

10000

100000

4.1 Kinetics of attachment of amino end-functionalized polystyrene (PSt: Mn = 4000) to epoxy functionalised surface (a) in good solvents (no segmental adsorption) and (b) in poor solvents (in the presence of segmental adsorption). (Adapted from Penn et al., 2004).

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Three-regime kinetics was observed in polystyrene tethering in the presence of good solvents when no segmental adsorption was registered (Fig. 4.1, curve (a)). Following rapid chemisorption over several minutes, the surface tethering density experiences a very slow increase proportional to log(time). In this second step, the new polymer chains approaching the surface diffuse on and through the tethered layer to a random location, thus giving rise to a spatially uniform increase of the surface tethering density. The third accelerating regime is explained by a spatially non-uniform increase of the surface attachment density through a cooperative mechanism. The second slow regime is diffusion-controlled and, as soon as the grafting density attains a certain value, the polymer chains, which are more relaxed when the grafting density is lower, start to be more and more stretched away and occupy a smaller horizontal surface. Thus, the chains from solution are allowed to touch the surface more freely, gradually giving rise to a greater increase in the tethering density (third regime). However, for poor solvents which generate segmental adsorption, only two-regime kinetics, which is characterised by a lower saturation time and a higher grafting density, were established for the same system due to an increased concentration of polymer chains at the surface (Fig. 5.1, curve (b)).

4.2.2 Adsorption parameters Free energy barrier and the strength of the binding interaction The polymer chain can be adsorbed on the surface through physical (physisorption) or chemical interactions (chemisorption). The height of the free energy barrier is one of the key parameters controlling polymer adsorption on surfaces. Physisorption and chemisorption are usually characterised, respectively, by low and high barriers. This is the result of the absence of significant activation energy in physisorption when compared to the high value of this parameter in chemisorption. In physisorption, the polymer–surface interaction may be of differing strengths. Weak physical interactions of the order of kT may prompt polymer desorption in the presence of a good solvent, when there is competition between the solvent and the polymer for surface adsorption sites. For example, polystyrene–polyisoprene block copolymers were found to de-wet silica gel when stored for several days (Leonard et al., 2002). A strong physisorption process, generally promoted by electrostatic or hydrogen bonding, is observed in polymer–surface systems where the binding interaction is higher than several kT (O’Shaughnessy and Vavylonis, 2003a; Panja et al., 2009). Under this condition, the desorption rates become very small. For chemisorption, the activation energy barrier is much higher than kT due to the high number of collisions of the monomer with the

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surface before the chemical bonding takes place (O’Shaughnessy and Vavylonis, 2003b). The high barrier for attaching to the surfaces which is encountered in chemisorption and in strong physisorption slows down the processes. Thus the adsorbed polymers have sufficient time to relax partially, causing the formation of loops which are composed of parts of the macromolecular chains. These are not in direct contact with the surface. In fact, chemisorption and physisorption processes can coexist in many polymer–surface interacting systems. The adsorption energy strongly influences the kinetics of adsorption. The adsorption time, t, was found to be proportional to N(1+2υ)(1+υ) or to N(1+υ) for weak or strong adsorption energies, respectively, where N is the length of the polymer chain and υ is the Flory exponent (Panja et al., 2009). Polymer concentration in bulk The influence of this parameter on polymer adsorption has been analysed by Bouchaud and Daoud (1987). Considering the adsorption of a flexible polymer from solution on a flat surface, they found four main regimes. (i) In a very dilute regime, insignificant excluded volume effects occur between the polymer chains, the surface coverage increases and the surface tension drops linearly with the bulk concentration. (ii) A more concentrated regime is characterised by the contribution of the excluded volume interaction to the free energy of the chains; the surface coverage increases only logarithmically with the bulk concentration, while the surface tension remains constant and the concentration profile extends up to a thickness of the order of the Flory radius. (iii) For bulk concentrations higher than the overlap concentration (the semi-dilute regime), the surface coverage remains almost constant and the surface tension shows a concentration dependence due to the distortion of the concentration profile near the surface. (iv) In a highly concentrated regime similar to the melt, the thermal energy becomes higher than the adsorption energy and blobs do not adsorb on the surface. Grafting density and solvent nature The grafting density is a parameter which exercises a strong influence on the topology of chemisorbed layers. In end-tethered polymers, the adsorbed layer topologies were found to depend on solvent quality and on grafting density. Under good solvent conditions, when the distance between neighbouring grafting sites is lower when compared to the radius of gyration of the polymer in its unperturbed state, the system may represent a combination of chemisorbed and physisorbed polymer segments (for attractive

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surfaces) (O’Shaughnessy and Vavylonis, 2003a). Alternatively, it may adopt a mushroom-shaped conformation (for non-adsorbing surfaces) (Koutsos et al., 1999). In a high grafting density regime, the polymer chains stretch away from the interface to avoid overlapping and form a uniform polymer brush. In a poor solvent, Koutsos et al. (1997) established that in low and medium grafting density, the polymer chains collapse separately or in a group, forming microphase-segregated clusters, respectively. With higher grafting densities, homogeneous layers are formed. The scaling and self-consistent mean field approaches predict that the equilibrium thickness (Le) of the solvent-swollen surface-bound layer will undergo an increase proportional to 1/3 of the power of the grafting density (σ) (relation 4.5) (Alexander, 1977; Milner et al., 1988): Le ∼ Lc σ1/3

[4.5]

where Lc is the contour length of the grafted chain. In other words, the film thickness is higher for layers with a higher grafting density due to the increased stretching of the polymer chains when compared with their undisturbed dimensions. However, in a high grafting density regime, when higher order interactions were considered, a variation of Le with σ n, where n increases with increasing σ, was established (Lai and Halperin, 1991). This theoretical prediction was confirmed by atomic force microscopy investigation of poly(methyl methacrylate) surface grafted layers of different grafting densities (Tsujii et al., 2004). For the highest investigated value (0.7 chains/ nm2), a large deviation from the predictions of Le dependence on σ established for low to moderate grafting densities (relation 4.5) was found, together with a layer thickness value close to that of fully extended polymer chains. The grafting density (σ) in a polymer layer is correlated with the layer thickness (h), the bulk polymer density (ρ) and the molecular weight of the polymer (Mn) by the expression (4.6), where NA is the Avogadro’s number (Luzinov et al., 2000): σ = (hρNA)/Mn

[4.6]

For quantitative characterisation of the transition point between a single grafted chain (mushroom shaped) regime and the polymer brush regime, Brittain and Minko (2007) used the reduced tethered density (Σ), defined as: Σ = σπRg2

[4.7]

where σ is the grafting density and Rg is the radius of gyration of a tethered chain under specific experimental conditions of solvent and temperature. Σ represents the number of chains that occupy an area which would normally

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be filled by a free non-overlapping polymer chain under the same experimental conditions. Three regimes were theoretically defined in the surface-tethered polymer layer formation for a monodisperse polymer: (i) the ‘mushroom’ or weakly interacting regime (Σ < 1); (ii) the crossover regime (Σ ≈ 1); and (iii) the highly stretched regime (Σ > 1). In real systems, due to the non-uniform statistical distances between grafting points and the polydispersity of the tethered chains, the transition between single grafted chains and a polymer brush is less well defined and large deviations from the theoretical predictions appear: i.e., a ‘true brush’ regime is found for Σ higher than 5. The Σ limits for the brush regime are strongly dependent on the tethered polymer and the nature of the surface as well as on the solvent. A surface grafted polymer layer was also described by the segment density profile of its surface attached chains (Φ(z)) (z is the distance to the surface on the perpendicular axis) and/or the height of the layer (h), which depends upon the graft density (σ), the polymer degree of polymerisation (N), as well as on the solvent quality. To characterise the polymer brushes, Alexander (1977) considered the balance of the interaction energy, Fint, between the statistical chain segments, and the elastic free energy (the energy difference between stretched and unstretched polymer chains), Fel, to give the free energy F of the chains (Eq. 4.8): F = Fint + Fel

[4.8]

For monodisperse polymers attached to a flat, non-adsorbing surface and having a low grafting density, he found a proportionality in the layer height h ∼ N × σ1/3 in good solvents and h ∼ N × σ1/2 in poor solvents. The simple Alexander model predicts a uniform distribution of polymer segments along h. However, Zhulina et al. (1991) and Wijmans et al. (1992) using the self-consistent field theories, and Lai and Binder (1992) using the Monte Carlo simulation, respectively, found a non-uniform, parabolic density profile. The theory of monocomponent systems was also developed for binary polymer brushes by Marko and Witten (1991). The morphology of the densely grafted polymer brushes, composed of polymers of different chemical structures and similarly high molecular weights, was found to be a result of the interplay between lateral and phase segregation (Zhulina and Balazs, 1996; Zhulina et al., 1996a, b; Minko et al., 2002; Minko, 2008). In a nonselective solvent, a ripple morphology is adopted as a result of the dominant lateral segregation, while in a selective solvent, the unfavoured polymers develop clusters embedded in the continuous phase of the favoured polymer (dimple morphology). Müller (2002) described a more general theory, based on a self-consistent field approach, which predicted lamellae, square, hexagonal and dimple morphologies.

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Nanocoatings and ultra-thin films

4.2.3 Topology of surface-bound layers Depending on the parameters discussed above, differing topologies of the adsorbed layers were found. Figure 4.2 shows the most representative structures of physisorbed and chemisorbed layers. The topologies of physisorbed single chains from diluted solutions are shown in Figs 4.2a–c as a function of the sorption mechanism, whereas Fig. 4.2d shows a representation of the hierarchical loops (the size increasing as the distance from the surface

–

–

+

+

(a)

+

(b)

– + –

–

+ –

+ – – +

– +

–

+

+

–

+

–

+

+ –

–

+

–

– +

–

–

– + –

+ –

–

(e) (c) Rg

N1/2 (f)

d

h

s*1/2 (d)

(g)

d

4.2 Topologies of adsorbed layers. (a–e), physisorbed layers: single chain from dilute (a–c) (O’Shaughnessy and Vavylonis, 2003a) and concentrated solutions/melt (d) (O’Shaughnessy and Vavylonis, 2003b): collapsed chain (a); accelerated zipped (b); zipped chains (c); hierarchical loop structure (d) (Aubouy et al., 1996); LBL films obtained by electrostatic interactions (e) (Hammond, 1999); (f, g), chemisorbed layers (Koutsos et al., 1999): mushroom conformation (f); brush conformation (g). Where s is the loop size; N is the number of monomer units of polymer chain; Rg is the radius of gyration; d is the distance between two neighbour binding sites; h is the layer height.

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Surface-initiated polymerisation for nanocoatings

87

increases), characteristic of polymer adsorption from concentrated solutions or melts. A simplified structure of films obtained by using layer-bylayer (LBL) deposition of two polymers with different electrically charged groups is shown in Fig. 4.2e. As can be seen from Fig. 4.2f,g, depending on the grafting density, the grafted layer can adopt two extreme topologies, i.e. the mushroom and brush-type layers. In the literature, the term ‘polymer brush’ is broadly used as a synonym for ‘tethered polymer layers’ and ‘end-grafted polymers’ without distinguishing real polymer brush structures from adsorbed polymer layers and from different types of chemically grafted layers. However, the term should be used only for systems which fulfil the defining terms of a polymer brush. Polymer brushes are defined as particular chemisorbed layers on planar or curved surfaces where the grafting density is higher than the polymer radius of gyration (Fig. 4.2g). The polymer chains are attached to the surface with a either a single or a small number of bonds. They are stretched away from the surface because of the excluded volume effects (Alexander, 1977; de Gennes, 1980; Milner, 1991; Halperin et al., 1992; Fleer et al., 1993; Szleifer and Carignano, 1996; Jones and Richards, 1999; Zhao and Brittain, 2000b). More recent comprehensive reviews on polymer brushes are published by Advincula et al. (2004) and by Edmondson and Armes (2009).

4.3

Preparation of surface-bound polymer layers

Polymer coated surfaces were obtained by both physi- and chemisorption processes (Fig. 4.3).

4.3.1 Polymer physisorption techniques Different techniques were used for polymer coating on surfaces, such as painting, spray, spin or dip coating, and doctor blading. All these procedures suppose the deposition of polymers from solutions, followed by solvent evaporation. If the solvent evaporation conditions are properly controlled, homogeneous layers with well-defined thickness can be obtained. More controlled techniques such as Langmuir–Blodgett or LBL deposition were also employed to coat different liquid or solid surfaces with polymers. Owing to the resultant weak physical interactions, the films can be detached under unfavourable conditions, e.g., desorption during solvent exposure, displacement by other molecules with a stronger affinity to the surface, dewetting (for film above the glass transition temperature) or de-lamination (for film below the glass transition temperature). There are only a few reports on polymer brushes obtained by physisorption (Ahrens et al., 1997; Belder et al., 1997; Piasta et al., 2009). The adhering

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88

Nanocoatings and ultra-thin films

Y

Y X

(a)

Y

X

X

(b) M

M

M M

M I

M M

M

M

(c)

X

Y

M I

M

M M

M1 M1

M1

I

M1

M1

M1 M

M I

M1

M

M1 M1 M1

M1 M

M

M1

M1 M1 M

(d)

4.3 Methods for preparation of surface linked polymer layers: (a) physisorption of (block co)polymers; (b) ‘grafting to’ approach; x, y = mutually reactive functionalities; (c) ‘grafting from’ approach; I = initiating species; M = monomer; (d) ‘grafting through’; M, M1 = identical or different polymerizing species.

polymers are composed of two different blocks; one block binds strongly to the surface through physical interactions, the other extends to form a polymer brush (Fig. 4.3a).

4.3.2 Polymer chemisorption processes To avoid the destruction of the polymer layers, a much stronger chemical bounding was proposed. Three main methods are considered for the preparation of chemisorbed polymer layers (Fig. 4.3b–d), i.e., ‘grafting to’, ‘grafting from’ and ‘grafting through’ approaches. In addition to these techniques, the possibility of building a solid surface inside the polymeric matrix was explored. A reversal of the method whereby the polymers are attached to a preformed surface, this bottom-up method, in which the surfaces are

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Surface-initiated polymerisation for nanocoatings

89

built in the presence of polymers, could be called coating by ‘reverse grafting’. The ‘grafting to’ approach This method involves pre-synthesised end- or chain-functionalised polymers. These polymers are attached to the solid surfaces through chemical linkages between the mutually interacting functionalities X and Y which are linked to the surface and the polymer chains, respectively (Fig. 4.3b). The stability of the whole structure is provided by the covalent bonds established between the substrate and the polymer chains. Different surfaces, such as silicon wafers, metals (gold), metal oxides or polymers, were covered with polymer brushes of various chemical structures using the ‘grafting to’ method (Table 4.1). Identical or mixed polymer grafts as well as hyper-branched polymers were attached to solid surfaces by using this method (Mori and Müller, 2004). The process was performed either in a solution (Viswanathan et al., 2005) or in a melt (Luzinov et al., 2000). The functional anchoring groups on the surface either already exist (e.g., OH groups on metal oxides) or are created by physical (e.g., oxygen or ammonia plasma treatment) or chemical methods. The most common method of activating the inorganic surfaces consists of the reaction with small functional compounds which results in the formation of self-assembled monolayers (SAM). Silanes, phosphates and phosphonates or thiols and disulfides were used to obtain SAMs on oxide surfaces, metals and metal oxides. Besides the functional groups, which provide an attachment to the surface, these compounds should also contain reactive groups, which will be available for reaction with the functional polymer. The ‘grafting to’ approach requires a sterically hindered migration of large polymer chains from solutions or melts to the surface. Thus, the process is diffusion controlled and the grafting density, σ, is quite low. This results in low surface coverage (Γ) and layer thickness (h) – usually in the tens of nm – as shown in equation (4.9) (where ρ is the polymer density) (Luzinov et al., 2000). Γ=h×ρ

[4.9]

The grafting density was found to increase proportionally with surface coverage and to decrease with the polymer molecular weight, Mn, for a constant Γ (Minko et al., 2003): σ = (6.023Γ × 100)/Mn

[4.10]

Other authors fitted their experimental results to a non-linear power law dependence of the number of chains grafted onto a surface unit (Gp) on the polymerisation degree (DP) of type Gp ∼ DP−δ, where δ = 0.4–0.6 (Luzinov et al., 2000) or 0.6–0.9 (Parvole et al., 2010).

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© Woodhead Publishing Limited, 2011

Gold

Glass slides

Silica particles

Carboxy-terminated PNV or PSt

Silicon wafer

PBd or P(St–Bd) prepared by anionic polymerisation PLMA, PDEGMA, PMMA, PNIPAAm, PSt prepared by RAFT PDMS functionalised with alkyl disulphide and perfluorinated side groups – PSt functionalised with quaternary ammonium or sulphonate groups, PDMAAm or PDMAAm–b–PMAPS Polyaniline nanofibers HS–PEGMA

P(TMSPMA–FDA) PMMA, PSt, PBuA terminated with N-succinimidyl ester groups

Deuterated PDMS containing monosilanol groups Anionically prepared PSt terminated with trimethoxysilyl groups PIbA prepared by RAFT

Anionically prepared PSt terminated with chlorodimethylsilane groups P(SiMA–EGMA) PSt–Ar(COOH)–PBuA

Functional polymer**

Surface nature*

Viswanathan et al., 2005 Nebhani et al., 2009

∼Si-OH + (CH3O)3 Si∼PSt

Sumerlin et al., 2003 Sawall et al., 2004 Arima et al., 2008

Au-S-C6H4-NH2 + polyaniline grown in solution Au + HS–PEGMA

Sun et al., 1996

Roth et al., 2009

Hübner et al., 2010

Au + HS∼polymer

1. ∼Si-OH + (MeO)3SiC3H6NHC2H4NHC6H4–CH=CH2 2. ∼Si- ∼ -CH=CH2 + (C6H4)SO2-C(=S)-S-PIbA ∼Si-OH + (MeO)3Si∼P(TMSPMA-FDA) 1. ∼Si-OH + (C2H5O)3-Si-(CH2)3-NH2 O 2. ∼Si∼NH2 + N OCO ~ polymer O 1. ∼Si-OH + Me2SiCl2, Me3SiCl, SiCl4 2. ∼Si-Cl + P(Li+ BD)− or Li+ P(St–Bd)− 1. ∼OH + Cl-SiMe2-C2H4-C6H4-CH2-N3 2. ∼Surface ∼ N3 + CH≡C-CH2-S-S-polymer Au + C5H11-S-S-C11H23-O-C3H6-PDMS

Pardal et al., 2009 Parvole et al., 2010

Luzinov et al., 2000 Minko et al., 2002 Tran and Auroy, 2001 Jon et al., 2003 Julthongpiput et al., 2003 Sirard et al., 2003

1. ∼Si-OH + (CH3O)3-Si-(CH2)3-O-CH2-epoxy 2. ∼epoxy + HOOC∼PSt (PNV) ∼Si-OH + Cl-Si(CH3)2∼PSt ∼Si-OH + (CH3O)3 Si∼P(SiMA–EGMA) ∼Si-OH + HOOC-C6H4 ∼ C6H3(OCO- PSt) (OCO-BuA) ∼Si-OH + HO∼PDMS

References

Surface binding reaction

Table 4.1 Surface-bound polymer layers obtained through ‘grafting to’ approach

© Woodhead Publishing Limited, 2011

Polyaniline

Yoshioka, 2009

Percec et al., 2009 Wang et al., 2007

Minko et al., 2003

Mayo et al., 2009

DurdureanuAngheluta et al., 2010 Jeon et al., 2010

∼Fe-OH + (C2H5O)3-R-PDMS R = C3H6-NH-CH2CH(OH)CH2-O-C3H6

1. CNT + 4-aminobenzoic acid (+ polyphosphoric acid + P2O5) 2. ∼CNT-CO-C6H4-NH2 + C6H5-NH2 Azido-terminated PSt or PMMA prepared -C6H4-O-CH2-C≡CH + N3-PSt (or PMMA) by ATRP Carboxy-terminated P(St–FSt) or PNV 1. Oxygen or ammonia plasma treatment 2. ∼C-OH or ∼C-NH2 + HOOC–P(St–FSt) (or PVP) DNA PPy-(C2H4COOH)(C2H4COO-NC4H4O2) + H2N-DNA 1. Al2O3-OH + Cl-Si(Me2)-C3H6-NH2 Carboxy-functionalised dendrimers 2. Alumina ∼ NH2 + G1,2 or 3-COOH (G1,2 or 3-COOH) PAAP particles -OH + (C2H5O)3Si-C3H6OCH2CH(OH)CH2-PAAP

PDMS

P(METMA–MES) PDMS–PEO–COOH

Toyoshima and Miura, 2009 Li et al., 2010 Pricop et al., 2010

Au + HS–P(Aam–GlyAAm) Gly = α-manoside or N-acetyl-β-glucosamine-O-C6H4Au + catechol-poly(METMA-MES) ∼Fe-OH + HOOC–PEO–PDMS

* CNT = carbon nanotubes; PPyCOP = electrocopolymerised carboxy and succinimidyl functionalised pyrroles; PTFE = polytetrafluoroethylene ** DNA = deoxyribonucleic acid; P(AAm–GlyAAm) = poly(acrylamide-glyco substituted acrylamide); PAAP = poly(amino-amide) particles modified with glycidoxypropyl triethoxysilane; PBd = poly(1,4-butadiene); PBuA = poly(butyl acrylate); PDEGMA = poly(diethylene glycol methacrylate); PDMAAm = poly(N,N-dimethylacrylamide); PDMAAm–b–PMAPS = poly(N,Ndimethylacrylamide) –block– poly{3-[2-(N-methylacrylamido)-ethyldiethylammonio] propane sulphonate; PDMS = polydimethylsiloxane; PDMS–PEO–COOH = polydimethylsiloxane-carboxy-terminated poly(ethylene oxide) graft copolymers; PEGMA = poly(ethylene glycol methyl ether methacrylate); PIbA = poly(isobornyl acrylate); PLMA = poly(lauryl methacrylate); P(METMA– MES) = poly{[(2-methacryloyloxy)ethyl] trimethylammonium chloride – (2-sulphoethyl methacrylate)}; PMMA = poly(methyl methacrylate); PNIPAAm = poly(N-isopropylacrylamide); PNV = polyvinylpyridine; P(SiMA–EGMA) = poly(trimethoxylsilylpropyl methacrylate –co– ethylene glycol methyl ether methacrylate); PSt = polystyrene; PSt–Ar(COOH)–PBuA = PSt and PBuA Y shaped copolymer bearing the arms connected through aryl ester junction; P(St–Bd) = poly(styrene –b– 1,4-butadiene) copolymers; P(St– FSt) = PSt–co–pentafluorostyrene copolymers; P(TMSPMA–FDA) = poly[(3-trimethoxysilyl)propyl methacrylate – stat – 1,1,2,2-tetrahydroperfluorodecyl acrylate]

ZnO particles

PPyCOP Nano-alumina

PTFE

CNT

Magnetite nanoparticles

P(AAm–GlyAAm)

92

Nanocoatings and ultra-thin films

The ‘grafting from’ method The ‘grafting from’ approach supposes the polymerisation of different monomers in the presence of appropriately functionalised surfaces, depending on the polymerisation mechanism (Fig. 4.3c). Because the polymer chains are grown from the initiating species (I) linked to the surface, this method is also called surface-initiated polymerisation. Unlike the ‘grafting on’ method, the ‘grafting from’ approach can yield a high grafting density, which provides a brush topology for the resultant polymer layer (Fig. 4.2g). The method allows anchoring distances of less than 3 nm and a greater thickness of layers (more than 2μ) of high molecular weight polymers. Surfaces which differ chemically (oxides, metals, polymers) and in shape (flat, curved or porous, nano/microsized) were first functionalised with initiating species by using chemical or physical activation methods and further used in situ to polymerise a large variety of monomers. The surfaces to be modified are either able to react with the appropriate functional compounds or need to be activated, e.g., by ion irradiation, oxygen or ammonia treatment, physisorption of functional polymers, etc. The attachment of the initiating species can be performed using the Langmuir–Blodgett technique or by self-assembling monolayer deposition and requires compounds which also bear specific functionalities able to interact with the functional groups of the surface. In order to functionalise silicon wafers, silica particles, glass, or metal oxides, halogeno- or alkoxysilane groups were used. Gold surfaces and particles were functionalised by reacting them with thiol-based compounds and so on. Almost all known polymerisation mechanisms – conventional radical polymerisation as well as controlled radical and ionic processes, metathesis, polycondensation or combination of two or more mechanisms – were employed to produce homopolymer, block copolymer or mixed polymer surface-linked layers. A rich literature on the preparation of surface-bound polymer layers by the ‘grafting from’ approach is available. To exemplify, in 2009, in Journal of Polymer Science: Part A: Polymer Chemistry each of the 24 issues contained at least one dedicated paper. It is practically impossible for this review to systematically cover all the work already done on finding the most appropriate means of producing surface-initiated polymer layers. Thus, Tables 4.2–4.4 show examples of surface initiating systems and the following comments highlight the specific behaviour of each of the mechanisms used.

Conventional radical polymerisation For conventional thermally or photochemically promoted radical polymerisation, azo-initiators, peroxides or photo-initiating groups were chemically

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Initiating surface**

Photopolymerisation Polystyrene latex

Microporous silica P(St- -VBCl-DVB) or P(MMA-VBCl- DVB) particles

-COO-C2H4-O -C6H4-CO-CMe2-OH

-O-Si-R-4,4′-azobis(4-cyanopentanoyl) groups Particle-CH2-S-C(=S)-N(C2H4)2

Polymerisation by thermal decomposition of surface bound initiating moieties Silicon wafer, glass -O-Si(Me)2-C3H6-NHCO-C2H4-C(CN)(Me)-N=N-C(CN) (Me)-C2H4-COOH -O- Si(Me)2-C3H6 -NHCO-C2H4-C(CN)(Me)-N=N-C(CN) (Me2) Silica gels -O-Si-(CH2)n-OCO-C2H4-C(Me)(CN)-N=N-C(CN)Me2 n = 3 or 11 Silicon wafer -OH...O=C-[poly(ε-caprolactone)-C2H4C(Me)(CN)N=N-C(Me)(CN)C2H4-...] (physisorbed macroepsidon-initiator) Silica -O-Si-C3H6-NHCO-C2H4-C(Me)(CN)-N=N=C(Me) (CN)-C2H4-COOH Silica nanoparticles -O-Si-(CH2)3-SH + AIBN Gold covered with PPy Au-PPy-N-C2H4-CONH-C(=NH)-CMe2-N=N-CMe2C(=NH)-NH2 Silicon wafer -O-Si-C3H6-OCO-C2H4-C(CN)(Me)-N=N-C(CN)(Me)-C2H4COO-C3H6-Si-O-

Surface*

Table 4.2 ‘Grafting from’ systems based on conventional radical polymerisation

AA NIPAAm

Murata et al., 2007

1. MβA+ MβASE 2. Functionalization with amines MMA HEMA, NIPAAm, EGDMA, DEAEMA

Guo et al., 1999; Guo and Ballauff, 2000; Ballauff and Lu, 2007 (Continued)

Ikeda et al., 2009 Ali and Mayes, 2010

Urzúa-Sanchez et al., 2002 Zhou et al., 2003 Roux et al., 2003

Prucker and Rühe, 1998a–c Stöhr and Rühe, 2000

Boven et al., 1990, 1991

Reference(s)

DBuPSt, DIPSt, DPPSt AN St

MMA, BuMA, HMA, LMA, SMA

St

MMA

Monomer***

© Woodhead Publishing Limited, 2011

-O-Si(Me2)-C3H6-OCO-C2H4-C(Me)(CN)-N=N- C(Me) (CN)-C2H4-COOC4H9 (living character)

ITO

Peng et al., 2009 Fundueanu et al., 2008

NIPAAm

Fulghum et al., 2008

MMA

1. NIPAAm 2. GMA 3. carboxylterminated PBuA VK

MMA

Geismann and Ulbricht, 2005; Geismann et al., 2007 Rahane et al., 2005, 2006 LeMieux et al., 2007

Hu et al., 2006

Reference(s)

* P(MMA–VBCl–DVB) = poly(methyl methacrylate-vinylbenzyl chloride-divinylbenzene); PPy = polypyrrole; P(St–VBCl–DVB) = poly(styrene–vinylbenzyl chloride-divinylbenzene) ** AIBN = 2,2-azobis(isobutyronitrile) *** AA = acrylic acid; AN = acrylonitrile; BuMA = butyl methacrylate; DBuPSt = 4-(di-t-butylphosphinyl)styrene; DEAEMA = 2-(diethylamino)ethyl methacrylate; DIPSt = 4-(diisopropylphosphinyl)styrene; DPPSt = 4-(diphenylphosphinyl)styrene; EGDMA = ethylene glycol dimethacrylate; GMA = glycidyl methacrylate; HEMA = 2-hydroxyethyl methacrylate; HMA = hexyl methacrylate; LMA = lauryl methacrylate; MβA = N-methacryloyl-β-alanine; MβASE = N-methacryloyl-β-alanine succinimide ester; MMA = methyl methacrylate; NIPAAm = N-isopropylacrylamide; PBuA = poly(butyl acrylate); SMA = stearyl methacrylate; St = styrene; VK = 9-vinylcarbazole

Redox initiated polymerisation Silica nanoparticles -O- Si-C3H6 -NH2 + Ce4+ (emulsion polymerisation) Pullulan microparticles Ce4+

Silicon wafer

-O-Si-C6H4CH2-S-C(=S)-N(C2H5)2 (photopolymerisation – iniferter approach) -O-Si-C6H4CH2-S-C(=S)-N(C2H5)2 (photopolymerisation (steps 1,2) and ‘grafting to’ (step 3))

AA, NIPAAm

-CONH-(C2H4-NH)n-H + benzophenone or benzophenone derivatives

Silicon wafer

HEMA

Benzophenone/FeCl3

Polypropylene porous membrane Poly(ethylene terephthalate)

Monomer***

Initiating surface**

Surface*

Table 4.2 Continued

© Woodhead Publishing Limited, 2011

Anchored initiating moiety**

-O-Si-C2H4-S-C(=S)-S-C6H5 (+AIBN) -O-Si-C2H4-S-C(=S)-S-CH(C6H5)-CO-OMe (+AIBN) O=P[C3H6-OCO-C2H4-C(Me)(CN)-S-C(=S)(C6H5)]3 (+AIBN) -O-Si- C3H6-OCO-C(Me)(R)-S-C(=S)-C6H5 (+AIBN)

Nitroxide mediated radical polymerisation (NMP) Bimolecular initiating systems Silica particles -O-Si-C5H10-O-CH2-C6H4-CH(Me)-O-N(t-C4H9)CH(C6H5)-CHMe2 + AIBN or -O-Si-C3H6-OCO-C2H4-C(CN)(Me)-N=N-C(CN) (Me)-C2H4-COO-C3H6-Si-O- + O-N (t-C4H9)CH(t-C4H9)P(=O)(OEt)2 Silica nanoparticles -O-Si-O-O-CMe3 + TEMPO 1. ∼Si-OH + (C2H5O)3-Si-(CH2)3-NH2 O 2. ∼Si∼NH2 + N OCO-C(Me2)O-N(t-C4H9)O C(t-C4H9)P(=O)(OC2H5)2

Silica nanoparticles

CdS quantum dots

Silica particles

Reversible addition-fragmentation chain transfer (RAFT) polymerisation Glass, silicon wafer -O-Si-C6H4-CH2-S-C(=S)-N(C2H5)2 (UV) Silica particles covered -O-Si-Polystyrene-CH2-CH(C6H5) -S-C(=S)-C6H5 (thermal initiation) with RAFT moiety terminated-polystyrene Silica nanoparticles -O-Si-C3H6-CH(Me)-S-C(=S)-C6H5 (+AIBN)

Surface*

Table 4.3 ‘Grafting from’ systems based on controlled radical polymerisation

Ni et al., 2006 Parvole et al., 2010

St Acrylic monomers, St

(Continued)

Parvole et al., 2002, 2003, 2004

Yang et al., 2009

Esteves et al., 2009

Zhao and Perrier, 2007

Li and Benicewicz, 2005

de Boer et al., 2000 Tsujii et al., 2001

Reference(s)

Acrylic monomers

St

1. BuA 2. St MA, BuA, St DMAAm, NIPAAm, MMA, St

MMA, St St

Monomer***

© Woodhead Publishing Limited, 2011

Anchored initiating moiety**

O

Silicon wafer

-O-Si-C3H6-OCO-CMe2-Br (water accelerated) -O-Si-C6H12-OOC-C3H6-Br (EtO)4Si + (EtO)3Si-C3H6-OCO-CMe2Br -O-Si-C3H6-NHCO-CMe2Br -O-Si-C11H22-OCO-CMe2-Br -O-Si-C2H4-C6H4-CH2-Br -O-Si-C2H4-C6H4-SO2Cl -O-Si-C11H22-OCO-CMe2-Br

Atom transfer radical polymerisation (ATRP) Silica (nano)particles -O-Si-C2H4-C6H4-CH2-Cl

Silica nanoparticles

Silica nanoparticles

~ Si ~ NH ~ CO-O-N

-O-Si-C5H10-O-CH2-C6H4-CH(Me)-O-N(t-C4H9)CH(C6H5)-CHMe2 1. M1 ‘grafting through’ with physical adsorption of M1 on carbon nanotubes in the presence of an alkoxyamine 2. NMP grafting of M2 on CNT-(M1)n -O-SiC3H6OCOCH[CH2C(Me2)(CN)] [ON(t-C4H9) CH(t-C4H9)P(=O)O(C2H5)2] O

Silica nanoparticles

CNT

-O-Si-C9H18-O-CH2-CH(C6H5)-O-N

Silicon wafer

Unimolecular initiating systems

Surface*

Table 4.3 Continued

OEGMA, MEMA, GMA,HEMA; MMA St, BuA, MMA NIPAAm ODVBzPhA AAm MAIpGlc

St, MMA

Ohno et al., 2005 Radhakrishnan et al., 2008 Wu et al., 2008 Czaun et al., 2008 Huang and Wirth, 1999 Ejaz et al., 2000; Ayres et al., 2008

von Werne and Patten, 1999 Perruchot et al., 2001

Chevigny et al., 2009

Bartholome et al., 2005

St St

Datsyuk et al., 2005

Blomberg et al., 2002

Husseman et al., 1999, 2000

Reference(s)

M1 = AA, St M2 = MA

St t-BuA St+VBCB or MaA

Monomer***

© Woodhead Publishing Limited, 2011

Silicon microchannels Silica membrane Pickering emulsion of silica nanoparticles Mica Gold

1. t-BuA 2. hydrolysis MesogeneMA

1. 2. 3. 4.

Au–S-C11H22-OCO-CMe2Br (1) Polymerisation of OEGMA Treatment of (1) with NaN3 Reaction with acetylene functionalised compounds

-O-Si-C2H4-OCO-CMe2Br -S-C11H22-OCO-CMe2-Br +1. HEMA; 2. DMAEMA (diblock copolymers) -S-C11H22-OCO-CR2R2-Br + 1. MA; 2. MMA; 3. HEMA R1 = alkoxy; R2 = H or CH3 (triblock copolymers) -S-C11H22-OCO-CMe2-Br

-O-Si-C11H22-OCO-CMe2-Br -O-Si-C3H6-OCO-CMe2Br -O-Si-C3H6-NHCO-CMe2Br

-O-SiMe2-C3H6-NHCO-CH(Me)Br

Ahn et al., 2004; Kim et al., 2005; Jung et al., 2006 Lee et al., 2007; Zheng et al., 2007

NIPAAm

(Continued)

Kim et al., 2002

MMA, MA, HEMA

OEGMA

Lego et al., 2008, 2009 Huang et al., 2002

Xu et al., 2005 Cao and Kruk, 2010 Chen Y et al., 2009

Cringus-Fundeanu et al., 2007; Ding et al., 2009

t-BuA HEMA, DMAEMA

t-BuA VDMA AAm DMAEMA, DEAEMA, DMAEA, BuAEMA HEMA St, MMA HEMA

Wu et al., 2007

St

-O-SiMe2-C2H4-OCO-CH(Me)Br

Liu et al., 2004

St, MA, FDA, t-BuA

-O-Si-C11H22-OCO-CMe2-Br (homo and block copolymers) 1. Physisorbed PGMA 2. Surface-O-CH2-CH(OH)-CH2-OCO-CH2-Br -O-Si-C11H22-OCO-CMe2-Br Grafting density gradients -O-Si-C6H12-OCO-CMe2-Br

Uekusa et al., 2007; Uekusa et al., 2009 Cullen et al., 2008a,b

von Werne and Patten, 2001; Ramakrishnan et al., 2002 Matyjaszewski et al., 1999

MMA

-O-Si-C3H6-OCO-CMe2-Br

© Woodhead Publishing Limited, 2011

DPhTA OEGMA, MMA 1. GAMA 2. 4-NBC St 1. MMA 2. BuMA St OEGMA+SIMA, OEGMA+MEtMA St, MMA DMAEMA St, MMA, BuA DMAAm MPEG-AAm

-S-C11H22- OCO-CMe2Br

-O-Si-C3H6-OCO-CMe2-Br -O-C6H3(-O-)-C2H4-NHCO-CHMeBr -O-Si-C11H22-OCO-CMe2Br

-O-Si-C3H6-NHCO-CMe2Br -O-Si-C11H22-OCO-CMe2Br

-OCO-CMe2Br -O-Si-C2H4-C6H4-CH2Cl

-C6H4-C2H4-Br -O-CH2-CMe2-CH2-OCO-CMe2Br -OCO-CMe2Br 1. Shell copolymerisation of styrene and 2-(methyl-2′-chloropropionato)ethyl acrylate 2. -COO-C2H4-OCO-CH(Me)Cl 1. Adsorption of anionic ATRP macroinitiator bearing sodium aryl sulfonate and bromine functionalities 2. Polymerisation in the presence of –OCO-C2H4-OCO-CMe2Br

Gold coated QCM chip

ITO Ti, stainless steel, TiO2 Ti

Layered silicate

Magnetite nanoparticles

CNT Carbon black Graphene oxide Polystyrene latex

MPC, GlMA, KSPMA, DMAEMA, NaStS

SBMA SPMA 1. HEMA 2. BuMA 3. HEMA OEGMA

-S-C11H22-OCO-CMe2-Br -S-C11H22-OCO-CMe2Br -S-C11H22- OCO-CMe2Br

Cationic Luxol sol

Monomer***

Anchored initiating moiety**

Surface*

Table 4.3 Continued

Kaiser et al., 2009a,b Gelbrich et al., 2010a,b; Marten et al., 2010 Matrab et al., 2006 Liu et al., 2006 Lee et al., 2009 Kizhakkedathu and Brooks, 2003; Kizhakkedathu et al., 2009 Vo et al., 2007

Wu et al., 2009 Karesoja et al., 2009

Ma et al., 2006; Chen X et al., 2009 Whiting et al., 2006 Fan et al., 2005, 2006 Raynor et al., 2009

Chang et al., 2008 Ramstedt et al., 2007 Rakhmatullina et al., 2009

Reference(s)

© Woodhead Publishing Limited, 2011

-NH-CH2-OCO-CMe2-Br

1. Radiation induced polymn.: electron beam activated surface 2. ATRP: N-bromosuccinimide/FeCl3(PPh3)2 -CONH-C2H4-OCO-CMe2-Br

-CH2-CH2-Br -Ph-CH2Cl -OCO-CMe2Br

Poly(styrene –co- 4-vinylbenzyl chloride) physisorbed -COO-C2H4-OCO-C(Me2)-Br

NIPAAm t-BuA NIPAAm+t-BuA HEMA, PEGMA

MMA

Jhaveri et al., 2007a

St, n-BuMA, t-BuMA, MMA+FlMA GMA PEGMA AA

Xu et al., 2007

Friebe and Ulbricht, 2007, 2009

Komatsu et al., 2010

Limé and Irgum, 2009 Yi et al., 2009 Pan et al., 2010

Mizutani et al., 2008

NIPAAm

* CNT = carbon nanotubes; DVB divinylbenzene; P(MMA/TriMA/MAEtBrBu) = nanoparticles obtained by copolymerization of MMA, 2-ethyl-2-(hydroxymethyl)-1,3-propanediol trimethacrylate and 2-methacryloxyethyl-2′-bromoisobutyrate; PPESK = poly(phthlazinone sulfone ketone) porous membrane; QCM = quarz crystal microballance ** AIBN = 2,2-azobis(isobutyonitrile); PGMA = poly(glycidyl methacrylate) *** AA = acrylic acid; AAm = acrylamide; BuA = butyl acrylate; BuAEMA = 2-(t-butyl-amino)ethyl methacrylate; BuMA = butyl methacrylate; DEAEMA = 2-(diethylamino)ethyl methacrylate; DMAEA = 2-(dimethylamino)ethyl acrylate; DMAEMA = 2-(dimethylamino)ethyl methacrylate; DMAAm = N,N-dimethylacrylamide; DPhTA = N,N-diphenyl-4-toluidyl acrylate; FDA = heptadecafluorodecyl acrylate; FlMA = fluorescein O-methacrylate; GAMA = 2-gluconamidoethyl methacrylate; GlMA = glycerol monomethacrylate; GMA = glycidyl methacrylate; HEMA = 2-hydroxyethyl methacrylate; KSPMA = potassium 3-sulfopropyl methacrylate; MaA = maleic anhydride; MA = methyl acrylate; MAIpGlc = 3-O-methacryloyl-1,2:5,6-di-O-isopropylidene-Dglucofuranose; MEMA = 2-(N-morpholino)-ethyl methacrylate; MesogeneMA = methacrylate ester containing azobenzene mesogene group; MEtMA = methoxyethyl methacrylate; MMA = methyl methacrylate; MPC = 2-(methacryloyloxy)ethylphosphoryl choline; MPEG-AAm = N-methoxy-poly(ethylene glycol) acryl amide; NaStS = sodium 4-styrensulfonate; 4-NBC = 4-nitrobenzoyl chloride; NIPAAm = N-isopropylacrylamide; ODVBzPhA = N-octadecyl-Na-(4-vinyl)-benzoyl-L-phenylalanineamide; OEGMA = methoxy-capped oligo(ethyleneglycol) methacrylate; SBMA = sulfobetaine methacrylate; SIMA = succinimidyl methacrylate; SPMA = sulfopropyl methacrylate; PEGMA = methoxy-capped poly(ethyleneglycol) methacrylate; St = styrene; VBCB = vinylbenzocyclobutene; VDMA = 2-vinyl-4,4-dimethyl azlactone

Nylon

Poly(ethylene terephthalate)

P(MMA/TriMA/ MAEtBrBu) particles DVB particles PPESK Regenerated cellulose membrane Polyethylene nonwoven fabric

Polystyrene

© Woodhead Publishing Limited, 2011

-O-Si-C6H4-CMe2-OCH3 + TiCl4 + (Me3C)2C5H3NH -O-Si-C11H22-C6H4-CMe2-OCD3 + TiCl4 + (Me3C)2C5H3NH -S-C9H18-OSO-CF3 -O-Si(Me2)-C3H6-C6H4-C(Me2)-Cl + TiCl4 + (Me3C)2C5H3NH

-OCO-NH-C6H3(Me)-NCO + sodium caprolactamate >Ph-C(C6H4)(C5H11)-Li >Ph-(CH2CH2-O)1–2-Li -OH

-S-C6H4-C6H4-Li -O-Si-C6H4-C(C6H4)(C5H11)-Li -N+(Cl−)(C2H5)2-C12H24-O-C6H4-C(C6H4)(C5H11)-Li -O-Si(Me2)-C11H22-O-C6H4-C(C6H4)(C5H11)-Li -S-C11H22-O-C6H4-C(C6H4)(C5H11)-Li

Initiating species

Coordination-insertion polymerisation, monomer activation by enzymes Gold and silica surfaces -S-C11H22-(O-C2H4)3-OH + Sn(octyl)2 -O-Si-C3H6-NHCO-(O-C2H4)2-OH + Sn(octyl)2 Gold -S-C11H22-(OC2H4)3-OH + lipase Silicon wafer, quarz; -O-Si-C3H6-NH2 -O-Si-C3H6-NH2 + Sn(octyl)2 glass fibres Carbon nanotubes >Ph-CH2CH2-OH + Sn(octyl)2 Magnetite nanoparticles -OCO-CH(NH2)-CH2-CH2-OH + Sn(octyl)2 stabilised with L-serine -OCO-CH2-OH + Sn(octyl)2 or glycolic acid

Gold nanoparticles Silica nanoparticles

Cationic polymerisation Silicate

Hydroxyapatite

Carbon nanotubes

Silica particles

Ionic polymerisation Gold Silicon wafer Clay Flat silica and gold

Surface

Yoon et al., 2003a,b, 2004 Yoon et al., 2003c Wieringa et al., 2001; Jiang et al., 2005 Priftis et al., 2009 Nan et al., 2009a,b

CLO NCA CLO CLO, EO CLO, LA

Jordan et al., 2001 Kim et al., 2004

Ox IB

DiOx

Zhao and Brittain, 2000a

Wiegand et al., 2008

Sakellariou et al., 2008

Yang et al., 2007

Jordan et al., 1999 Quirk and Mathers, 2001 Zhou et al., 2001 Advincula et al., 2002

Reference(s)

St

St, CLO LA

St Is St St or 1. St; 2. Is or 1. Bd; 2. St CLA

Monomer*

Table 4.4 ‘Grafting from’ of unsaturated and cyclic monomers by using ionic, metathesis and other polymerisation mechanisms

© Woodhead Publishing Limited, 2011

-O-Si-(5-bicycloheptenyl or norbornenyl) Ru[(cyclohexyl)3P]2Cl2

1. -O-Si-C3H6NH2 2. O-Si-C3H6-NH=C(NH-C4H9)[Ni(CH=N-C4H9)3]2+2ClO4− PE + thioxantone + UV light PTFE-OOH through ion irradiation -CONH-C6H3(Me)-NH2 -S-alkyl-NH2 -O-Si-alkyl-NH2 1. -O-Si-C6H4-CMe2-OCD3/TiCl4 2. -O-Si-C6H4-CMe2-PSt-Cl/CuBr/anisole 1. H3N+(Cl−)-C6H12-OCO-C(Me2)-Br 2. H3N+(Cl−)-C6H12-OCO-C(Me2)-PSt-Br/AgPF6

Silicon wafers Quarz Low density PE PTFE Carbon nanotubes Gold Silica Silicate

Zhao and Brittain, 1999

1. 2. 3. 4.

St (cationic) MMA (ATRP) St (ATRP) THF (cationic)

Bai et al., 2009 Choi et al., 2009 Lafuente et al., 2009 Tuberquia et al., 2009

GMA AA FD DAM

Yenice et al., 2009

Ogawa et al., 2007 Lim et al., 2008

Vestberg et al., 2007

Feng et al., 2007; Harada et al., 2003; Jeon et al., 1999; Rutenberg et al., 2004

1. DeMPA-N3 2. DeMPA-C≡CH a.s.o DISPB + DEMTEB ICAME

NOR BCH COD

* AA = acrylic acid; BCH = 5-bicycloheptadiene; Bd = butadiene; CLA = ε-caprolactam; CLO = ε-caprolactone; COD = cyclooctadiene; DAM = diazomethane; DeMPA-C≡CH are alkyne functionalised dendrimers based on 2,2-bis(methylol)propionic acid; DeMPA-N3 = azide functionalised dendrimers based on 2,2-bis(methylol)propionic acid; DiOx = p-dioxanone; DEMTEB = 1,4-diethynyl-2,5bis(methoxytriethoxy)benzene; DISPB = 2,5-diiodo-1,4-bis(3-sulfonatopropyl)benzene; EO = ethylene oxide; FD = 1,10-bis[94-formyl3-hydroxyphenyl0oxy]decane; GMA = glycidyl methacrylate; IB = isobutene; ICAME = L-isocyanatoalanyl-L-alanine methyl ester; Is = isoprene; LA = lactides; MMA = methyl methacrylate; NCA = N-carboxyanhydrides of γ-benzyl L-glutamate and γ -methyl L-glutamate; NOR = norbornene; Ox = 2-oxazolines; St = styrene; THF=tetrahydrofuran

Clay

-O-Si-C3H6- NHCO-C6H4-I + Pd(PPh3)4 +CuI + NEt3

Silica microparticles

Other mechanisms and combined mechanisms Silicon wafer -O-Si-C3H6- NHCO-C2H4-C≡CH

Metathesis Inorganic surfaces

102

Nanocoatings and ultra-thin films

C(Me)(CN)-N=N-C(Me(CN)-R

CN

CN

hν or Δ

+

C

–N2

Me

C-R Me

Bulk polymerisation

(a)

Photoexcitation Ph-CO-Ph [Ph-CO-Ph]*

(b)

Ph

+ C Hydrogen Ph abstraction OH Low reactive Surface initiating species radical

4.4 Surface initiated conventional radical polymerisation: (a) scission of surface-bound azo labile groups and promoting the side bulk polymerisation; (b) polymerisation in the presence of photosensitisers.

attached to solid substrates. The peroxide initiators were also introduced directly, by physical methods. Table 4.2. shows such polymerisation systems. The shortcomings of this procedure arise from the fairly high polydispersity of the resultant polymers and the formation of bulk polymer, as the thermal or photochemical scission of the initiator gives raise to two active radical species, only one being bound to the surface (Figure 4.4a). The bulk polymer will entangle with the surface-linked polymer or will adsorb on the surfacebound layer, thus requiring a careful post-polymerisation cleaning. Moreover, crosslinking may occur because of the termination of the chain growth through the recombination. To minimise bulk polymerisation, photosensitisers such as benzophenone were used (Hu et al., 2006). Under a photo-excited state, benzophenone is able to abstract a hydrogen from the substrate, yielding a non-reactive ketyl radical and an initiating radical on the surface (Figure 4.4b). However, a more controlled radical polymerisation in the presence of surface-linked azo-initiators may be achieved when the surface is not flat but is porous (Ikeda et al., 2009), due to the reduced termination reaction determined by restriction of the movement of the polymer chains covalently bound to the inside walls of the porous membrane. Controlled radical polymerisation Living radical polymerisation mechanisms such as reversible addition– fragmentation chain transfer polymerisation (RAFT), nitroxide mediated radical polymerisation (NMP) and atom transfer radical polymerisation

© Woodhead Publishing Limited, 2011

Surface-initiated polymerisation for nanocoatings

103

(ATRP) of vinyl monomers, (meth)acrylates or macromonomers (Table 4.3), as well as ionic polymerisation of unsaturated and cyclic monomers and methatesis, are alternative applications for achieving a better control of the surface-bound polymer layers (Table 4.4). Owing to their living character, these approaches are also appropriate for the preparation of surface grafted block copolymers (Boyes et al., 2004; Minko, 2008). RAFT polymerisation requires chain transfer compounds bearing a leaving and re-initiating R group and a stabilising Z group [i.e., Z–C(= S) S–R]. The surface bounding of the chain transfer agent can be performed either from R side (R-group approach) or from Z side (Z-group approach) of RAFT molecule (Figure 4.5a). The polymerisation is initiated by thermal or chemical (AIBN) methods. The chain propagation involves the dynamic equilibrium between the propagating and dormant radicals (Figure 4.5b). RAFT polymerisation supposes fairly simple reaction conditions, similar to those of the conventional radical polymerisation performed in the presence of chain transfer agents. The properties of the resultant surface-bound polymer layers are strongly dependent on the manner in which the RAFT species are linked to the surface. While polymerisation in the presence of RAFT molecules which are surface-linked via the leaving and re-initiating R group is similar to the ‘grafting from’ approach, the other binding method provides similar conditions to the ‘grafting to’ approach. Thus the R-approach allows the preparation of polymer surface layers with high grafting densities of high molecular chains but of broader molecular weight distribution, as a consequence of the chain coupling. The differing Z-group approach gives rise to well-defined grafted polymers, with monomodal molecular weight distribution, while the grafting density is lower as a result of the shielding effect. To provide controlled surface-grafted polymers, a free RAFT agent should be added into the polymerisation system. The surface radical migration effect (Tsujii et al., 2001), which determines the recombination of the propagating chains, can be minimised by decreasing

S Z C

S S R

R S

C

Z

(a) S C Z (b)

S + Pn S Pm

S

Pn

C Z

Pn

C S Pm

Z

+

Pm

S M

M

4.5 RAFT polymerisation: (a) surface mediated Z and R approaches; (b) reversible addition-fragmentation chain transfer.

© Woodhead Publishing Limited, 2011

104

Nanocoatings and ultra-thin films

the surface density of the RAFT species and by using low initiator/chain transfer agent ratios. Barner-Kowollik and Perrier (2008) have recently highlighted the advantages and challenges of RAFT polymerisation. The preparation of polymer-coated surfaces by nitroxide-mediated polymerisation (NMP) has been recently reviewed (Ghannam et al., 2006). Chemically differing alkoxyamine functionalities were linked to planar and particle surfaces to further promote the controlled radical polymerisation of different vinyl and acrylic monomers (Table 4.3). Bimolecular and unimolecular initiating systems were described (Figure 4.6a). The propagation of the polymer chain takes place through activation–deactivation processes involving a reversible combination of the growing chains with nitroxide radicals (Figure 4.6b). As in RAFT polymerisation, obtaining high grafting densities with polymers of high molecular weight and controlled polydispersity requires some free alkoxyamine to be sacrificed. Practically all monomers can be polymerised by this method, including those which are not suitable for other mechanisms. However, not all surfaces can be grafted

(a) initiating systems Unimolecular initiating systems O R''' X R O C

R'

C

O

N

R1 R2

R'' Bimolecular initiating systems O X R

O C C2H4 N N C2H4 R' + O

X R'' O R''' O

N

R1 R2

R2 CN

CN + CH3 C

R1

N

N

N

CH3

C CH3 CH3

X = anchoring group R, R', R'' = hydrocarbonate radicals R''' = hydrocarbonate radical or succinimidyl ester R1 = hydrocarbonate radical R2 = phosphonate substituted radical (b) chain propagation polymer

O

N

R1

ka

R2

kd

polymer +

O

N

kp monomer

4.6 Nitroxide-mediated surface grafting.

© Woodhead Publishing Limited, 2011

R1 R2

Surface-initiated polymerisation for nanocoatings

105

due to the fairly high reaction temperature. Grafting densities as high as 0.9 chains/nm2 were obtained by using nitroxide SG1 bearing N-succinimidyl ester groups (Parvole et al., 2010). ATRP is by far the most widely used method of preparing grafted polymers and copolymers on either flat or curved surfaces (Table 4.3). It is based on the dynamic exchange between halogen-terminated growing chains/ Cu(I)XL2 (dormant species) and macro-radical/Cu(II)X2L2 complexes (active species) (Figure 4.7), where X and L are halogen and amino ligand, respectively. The chain propagation and termination are first-order and second-order reactions, respectively. The equilibrium in Figure 4.7 is usually strongly shifted to the left, the concentration of the free radicals is low (10−7, 10−8 mol/L) and the concentration of Cu(II) should be as high as 5% of that of Cu(I) to ensure good control of the process. Thus, the termination reaction is diminished and all the chains grow simultaneously, producing polymers of low polydispersity. The amount of surface-bound initiator is too low, especially in flat surface-initiated polymerisation, to provide the requested Cu(II) concentration. To overcome this shortcoming, the addition of free (‘sacrificial’) initiator and of the necessary amount of Cu(II) at the beginning of the polymerisation is imposed. The addition of the ‘sacrificial initiator’ provides sufficient concentration of the persistent radicals and control of the degree of polymerisation. It also permits an easy determination of the polymer molecular weight by using the free polymer formed in solution. The grafting density which affects the tethered polymer conformation and the thickness of the polymer layer also depends on the surface coverage by the initiator, which can be modulated by attaching active and inactive coupling agents to the surface. High grafting density and an easier control of the thickness of the polymer layer can be achieved in particle-initiated polymerisation due to the large curvature of the surface and to the ease with which the ratio between the bound initiating species and the monomer concentrations can be varied. The surface-initiated ionic polymerisation was performed in the presence of surface-attached cationic or anionic catalytic moieties. Both unsaturated and cyclic monomers were polymerised in the presence of different types of activated surfaces. As in other surface-initiating systems, the active

Pn X / Cu(I) X L2

k1 k2

Pn / Cu(II) X2L2 k3 M

4.7 Mechanism of surface ATRP propagation.

© Woodhead Publishing Limited, 2011

106

Nanocoatings and ultra-thin films

species are attached to the surface mainly by the self-assembly monolayer technique. Ring opening methatesis, polycondensation, click chemistry or combined mechanisms were also used to prepare surface grafted polymers on different inorganic surfaces by surface-initiated polymerisation. Table 4.4 shows some recent results. The ‘grafting through’ method The ‘grafting through’ method was used less in preparation of surfaceattached polymer layers. However, due to the simplicity of the method, some trials have been performed recently, either to modify the properties of flat surfaces, or to increase the colloidal stability or compatibility of inorganic particles. This approach supposes the linking of a polymerisable moiety to the surface (usually through self-assembling monolayers), followed by the copolymerisation of the activated surface with an appropriate monomer (Fig. 4.3d). Polymers such as poly(meth)acrylates, poly(vinyl acetate/pivalate), water-soluble or photo-active polymers were bound to inorganic or polymer surfaces by this method using conventional free radical polymerisation, ATRP or polycondensation reactions (Table 4.5).

4.3.3 Nanocoating by ‘reverse grafting’ This approach is designed to provide solid inorganic particles of nanometric dimensions covered with thin layers of polymers (see Fig. 4.8). The most challenging case is concerned with the in situ preparation of a stable polymer–metal nanoparticle solution. This so-called bottom-up method employs chemical approaches to the assembly of nanoparticles from either mononuclear metal ions or structures with a lower inclination to nucleation. The synthesis and stabilisation of nanoparticles in a polymeric solution takes place in several stages, among which the main ones are: (i) nucleation and formation of the new phase; (ii) formation of the stable polymer–metal nanoparticle complexes. Nucleation and phase formation The formation of nanoparticles from a single metal atom and their conversion into a compact metal proceeds through the generation of intermediates such as clusters, complexes and aggregates. As the number of atoms increases, a fairly stable state is reached. At a certain point, the mean frequency of an atom attaching to the ensemble becomes equal to the mean frequency of separation and this makes unproductive further joining of

© Woodhead Publishing Limited, 2011

Surface-initiated polymerisation for nanocoatings

107

Table 4.5 Surface-bound polymer layers obtained by ‘grafting through’ approach Monomer functionalised surface*

Polymerising monomer**

Initiator***

Reference(s)

Silicon wafer-O-Si-CH=CH2

VAc

AIBN

Poly(VUD-co-EGDMA)-OCOC(Me)=CH2 Maghemite(-O)2P(=O)-[OC(Me)-CH2]5-OCOC(Me)=CH2 -O-Si-C3H6-NHCO-OCH2-C≡CH

VP

AIBN

GMA

AIBN

Nguyen et al., 2003 Nguyen et al., 2007 Tocchio et al., 2009

5-decyne

Silicon wafer-(O-Si-C3H6)2 (9,9′-dibromofluorene) Mesoporous silica nanoparticles-O-Si-CH=CH2 Magnetite-O-Si-C3H6-OCOC(Me)=CH2 Magnetite-O-Si-C6H4-CH2-Cl

DBrDHeF

WCl6/Ph4Sn + microwave Ni(0)

Ethylene

Ni(COD)2 / EtPA AIBN CuBr/amine

NIPAAm OEGMA

Jhaveri et al., 2007b Jhaveri et al., 2009 Wei and Zhang, 2009 Frickel et al., 2010

* Poly(VUD-co-EGDMA) = poly(vinylundecanoate-co-ethylene glycol dimethacrylate) particles ** DBrDHeF = 2,7-dibromo-9,9-dihexylfluorene; GMA = glycidyl methacrylate; NIPAAm = N-isopropylacrylamide; OEGMA = oligo(ethyleneglycol) methylether methacrylate; VAc = vinyl acetate; VP = vinyl pivalate *** AIBN = 2,2′-azobis(isobutyronitrile); EtPA = ethyl-4,4,4-trifluoro-2(triphenylphosphoranylidene)-acetoacetate; Ni(COD)2 = bis(1,5-cyclooctadiene) nickel

Metal salt Nucleation Polymer Solid phase formation

Particle growth

Polymer–metal complex Nanocoated metal particle

4.8 Nanocoating by ‘reverse grafting’.

© Woodhead Publishing Limited, 2011

108

Nanocoatings and ultra-thin films

metal atoms to the aggregate. Such structure is called the critical nucleus of the new phase and its dimensions are below 10 nm (Abraham, 1974; Morokhov et al., 1977; Villuendas and Bowles, 2007). The nucleation of a new phase is critical to the formation of an interface within the bulk of the polymeric phase. This interface binds the amount of critical nuclei which are capable of progressive spontaneous growth (Barret, 1973; Morokhov et al., 1977; Gibbs, 1993; Anisimov, 2003; Fokin et al., 2005). Homogeneous nucleation is a process characterised by the appearance of new phase nuclei within a metastable homogeneous system (Abraham, 1974). This can be referred to as transition of the substance into the thermodynamic stable state through a sequential reversible association. During nucleation, elements of the interface, and the interfaces themselves, first emerge and then disappear. Heterogeneous nucleation is a process during which the interactions with the formation of new phase nuclei are running in contact either with heterogeneities found in the generating phase, or with the surface. Heterogeneous nucleation occurs in multicomponent systems with spatial inhomogeneities at the interface (substrates, including polymer substrates, extraneous inclusions and surfaces, additives or crystalline particles already formed) (Fokin et al., 2005; Villuendas and Bowles, 2007). The nanoparticles formed on the polymer surface by heterogeneous nucleation have specific features related to the surface energy anisotropy at the crystalline nucleus– surface–medium interface. The general picture of nucleation and growth of the new phase during chemical reactions includes a set of numerous interrelated processes as follows. (i) Chemical reactions which can be thought of as the source of building material for the new phase. These may consist of one or more reactions with the participation of one or several reagents and may proceed on the surface or within the bulk. During chemical transformations, highly reactive particles capable of condensation reach a certain local concentration at which they associate and form a new phase. (ii) The mass transfer processes of reagents into the reaction zone (if few compounds take part in the reaction), the movement of products of chemical interaction capable of aggregation to the condensation zone, or the removal of some products of reaction which do not participate in crystallisation in the zone, in case new particles grow on the surface, particularly that of the polymer. (iii) Sorption processes which could make a contribution to nucleation and growth of the new phase particles. The processes are observed in adsorption of synthesised particles, of reagents or products of interaction on the surface of growing nascent clusters and in desorption from the surface. Chemisorption interactions on the growing particle surfaces are interrelated with the processes of stabilisation.

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Formation of polymer–nanoparticle complex The reduction of metal ions in polymer solutions results in formation of metal nanoparticles with a narrow size distribution and a mean size of less than 10 nm. It is known that shielding of the growing metal nanoparticle by macromolecules (the formation of polymer–metal complexes) will finally be followed by the growth step (Hirai and Toshima, 1986). Therefore, the size distribution and mean size of the particles in the resulting solution can be defined by the dependence of the probability of the complex formation on the size of the growing metal particles. This probability was assumed to be defined by thermodynamic stability of the corresponding complex (Litmanovich et al., 2010a, b). It was suggested (Papisov and Litmanovich, 1999; Litmanovich et al., 2010b) that the interactions between very small particles and long macromolecules are similar to those between oligomers and polymers and, for this reason, both can be treated using the model of adsorption of small species on a long polymer chain. The results of these interpretations have several important consequences (Papisov and Litmanovich, 1999): (i) at low concentrations of protective polymer and nanoparticles with diameters within 1–10 nm, a high stability of polymer– nanoparticle complexes can be provided for systems having ΔG values as high as a few percent of the specific surface energy values for common solids; (ii) the stability of polymer–nanoparticle complexes strongly depends on the size of the nanoparticles; (iii) the macromolecules–nanoparticles interactions are very selective in regard to both the macromolecular structure and the size of the particles. These conclusions explain and predict the features of the new phase formation processes which are taking place in the polymer solution. The thermodynamic stability of the polymer–metal nanoparticle complex depends on the particle’s surface area only when the polymer chain is able to shield the entire surface of the particle. If either the chain is too short or the particle is too big, only a part of the surface area can be shielded and the stability of the complex will depend on the length of the chain. In fact, a stable polymer–metal nanoparticle solution should be considered to be a fine dispersion of micelles which represent the polymer–metal complexes formed due to the cooperative non-covalent interaction between macromolecules and the surface of the metal nanoparticles. Polymer chains envelope the metal nanoparticles, thus lyophilising and protecting them from aggregation or oxidation. The nature of the interaction and the structure of the polymer–metal complexes are under continuous investigation. It is generally supposed that the hydrophobic interaction between the macromolecular chains and hydrophobic surfaces of metals plays an essential role in protection (Hirai and Toshima, 1986). Arguments supporting these assumptions have been presented in reports describing the interaction between

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copper nanoparticles and poly(N-vinyl-caprolactame) (Hirai and Yakura, 2001) or the coulomb interactions between oppositely charged metal nanoparticles and polymer chains (Ostaeva et al., 2006). The interaction between the macromolecular chains and the nanoparticles is reversible. The probability of a complex formation (often described as the mutual recognition between a growing particle and a macromolecule) rapidly increases from zero to unity in a narrow interval of the particle’s diameter. The recognition is followed by the shadowing of the particles and the cessation of their growth. The mutual recognition term (the probability of the formation of polymer–metal nanoparticle complexes) indicates that the polymer–metal nanoparticle should be considered as a specific, and sometimes unique, dual system built with a polymer having a specific chemical structure and a metal nanoparticle which is precisely defined, both in its nature and size. Recent work has offered strong arguments concerning the specificity of the polymer–metal nanoparticle dual systems (Sacarescu et al., 2010). According to these reported results, the polyhydrosilanes develop specific interactions with the silver nanoparticles which are synthesised in situ, resulting in weak charge transfer complexes located along the macromolecular chains. These structures represent the nucleation centres for the growth of the silver nanoparticles. Therefore in the first stage of the process, the polymer represents an active template which binds the silver atoms and permits the formation of metal nanoparticles. In the second stage, the particle growth and increase in size result in the enveloping and protection of the surface, thus producing a highly stable polymer–nanoparticle system.

4.4

Properties and applications

4.4.1 Physicochemical properties of surface-bound polymer layers and comparison to bulk properties The surface-bound polymer layers showed significantly different physicochemical properties when compared to the bulk polymers of identical structure and similar molecular weight. The glass transition temperature (Tg), the elastic behaviour and miscibility are strongly influenced by the nature and characteristics of the polymer backbone, and also by the type of surface binding and grafting density. Tg of the surface-bound layers was found to depend on the film thickness and on the nature of the surface (Prucker et al., 1998; Fryer et al., 2001). Moreover, Tsujii et al. (2004) established significant differences in the variation of Tg for surface-tethered poly(methyl methacrylate) layers with low, medium and high grafting densities (brush regime) and cast films of the same molecular weight. The Tg of the brush layer decreases, while that of

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the cast film increases up to a thickness limit of about 60 nm, due to the surface effects. At higher thicknesses, both layers have an independent variation of Tg on the thickness, but the Tg of the brush film was 8 °C higher than that of the cast film. This differing behaviour of films which are adsorbed and highly grafted on the surface (not observed in low and medium grafting densities) is explained by the anisotropic structure and conformation of the brush layer. The elastic properties of the polymer layers on the surface also depend on the nature of the binding. Urayama et al. (2002) explained the lower compressibility of the polymer brushes when compared to the cast films, by the strain-hardened effect of the highly stretched and entangled chains. Polymer layers of low and medium grafting densities are miscible with polymers of similar structure, while the highly stretched polymer brushes are not (Tsujii et al., 2004). The surface-bound polymers also showed a different responsiveness to specific stimuli when compared to the unbound polymers. For example, pH-sensitive brushes, such as high density surface-tethered poly(methacrylic acid), present a significant increase of the pKa value (about 10) when compared to the bulk polymer (pKa about 4–5) (Tsujii et al., 2004). Poly(Nisopropylacrylamide) (PNIPAM), a thermosensitive polymer with a lower critical solution temperature (LCST) of about 32 °C, when tethered to a surface (a thickness of dry film of about 50 nm) does not present a sharp drop to the LCST, but a continuous transition of between 10 and 40 °C, suggesting the existence of partially collapsed PNIPAM chains at room temperature (Balamurugan et al., 2003). The same authors observed a sharp increase of the advancing contact angle at 32 °C, concluding that the outermost region of the brush remain highly solvated, while less solvated segments within the polymer brush undergo dehydration and collapse over a broad range of temperatures.

4.4.2 Properties and applications of modified surfaces Adhesion is a fundamental property of modified surfaces and affects both surface and interface properties. Raphaël and de Gennes (1992) and Ji and de Gennes (1993) were the first to investigate the adhesion properties of surface layers by modelling adhesion between two rubber surfaces having brush-type layers extending from one surface to another across their interface. The adhesion was found to be a function of the sum of the thermodynamic work of adhesion between elastomeric surfaces in the absence of the polymer brush and of the energy needed to pull out the connecting polymer chains from one layer. For surface-bound functional polymers, O’RourkeMuisener et al. (2003) used the Scheutjens–Fleer self-consistent mean-field theory to demonstrate that polymers with adjacent low-energy functional

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groups located at one end of a chain show an optimum low energy release surface, whereas an optimum high energy adhesion surface is obtained by the introduction of adjacent high energy functional groups in the middle of a polymer chain. Both low and high adhesive surfaces are of great interest for bio-applications such as artificial implants, cell cultures and tissue engineering (Caster, 2004; Ayres, 2010) as well as for nanotechnology (Luzinov et al., 2004; Ayres, 2010). Surfaces of switchable properties contain either stimuli-responsive homopolymers or copolymers composed of incompatible blocks or grafts. Remarkable switchable properties were also found in mixed polymer (Minko, 2008). The changing of surface structures and properties by the action of selective solvents, pH or temperature (Jiang and Li, 2009; Lenz et al., 2010) is of great interest for bio-applications. The surface properties of biomaterials and biosensors, etc. can be modulated on demand, providing dynamic control over surface–biomolecule interactions such as protein adsorption–desorption (Balamurugan et al., 2005; Cole et al., 2009; Bucatariu et al., 2010) or cell attachment–detachment (Hyun et al., 2004; Canavan et al., 2005a,b; Cheng et al., 2005; Tsuda et al., 2005; Smith et al., 2005). Stimuli-sensitive surfaces were also proposed as appropriate materials for the preparation of different catalytic systems (Li et al., 2008; Jiang et al., 2009; Marten et al., 2010).

4.5

Acknowledgement

One of the authors (Maria Butnaru) acknowledges the financial support of European Social Fund – ‘Cristofor I. Simionescu’ Postdoctoral Fellowship Programme (ID POSDRU/89/1.5/S/55216), Sectoral Operational Programme Human Resources Development 2007–2013.

4.6

References

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5 Methods for analysing nanocoatings and ultra-thin films D. M. BASTIDAS, M. CRIADO and J.-M. BASTIDAS, National Centre for Metallurgical Research (CENIM), CSIC, Spain

Abstract: Linear potential sweep and impedance measurements are methods frequently used for studying corrosion behaviour, mass transport processes and the protective properties of coatings applied on metal substrates. Surface-sensitive analytical techniques are useful for understanding the composition and structure of ultra-thin film coatings. Among these techniques, atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), specular reflectance infrared, Raman and Mössbauer spectroscopies supply excellent results. Ion spectroscopy and glow discharge optical emission spectroscopy provide information about the concentration and the depth of the elements on the surface of a coating. Finally, electron microscopy provides morphologic and topographic information about the surface of solids. Key words: cathodic stripping, electrochemical impedance spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy, ion spectroscopy, electronic microscopy.

5.1

Introduction

This chapter deals with methods used in the study of corrosion behaviour, mass transport processes and the protective properties of coatings applied to metal substrates. Linear potential sweep and impedance measurements have been used to study the relationship between current and potential peaks for copper specimens subjected to different tarnishing treatments. Electrochemical methods can be used to provide a reasonable approximation of changes in the dielectric constant caused by water absorption and the pigment/polymer proportions and porosity of organic coatings. Surfacesensitive analytical techniques are useful for understanding the composition and structure of ultra-thin film coatings. Techniques that supply excellent results in this area include atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), specular reflectance infrared, Raman and Mössbauer spectroscopies. Moreover, ion spectroscopy and glow discharge optical emission spectroscopy make it possible to obtain information about the concentration and the depth of 131 © Woodhead Publishing Limited, 2011

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the elements present on coating surfaces. The final technique discussed is electron microscopy, which provides morphologic and topographic information about the surface of solids.

5.2

Electrochemical methods

5.2.1 Potentiodynamic potential/current measurements Linear potential sweep is a method commonly used for studying the films formed on metallic surfaces. Using potentiostatic transients as a basis, Müller (1931) proposed that a number of metals can be passivated by the formation of an insoluble film on the metallic surface. This film can nucleate at several points and then spread laterally across the metallic surface. It is known to be useful to evaluate the potentiodynamic potential/current density relationships for a film formation process under ohmic resistance control (Bastidas et al., 1997). Copper and its alloys are widely used in many environments. On exposure to the atmosphere, clean copper transforms from salmon-pink to a progressively darker brown. The natural green film that forms on copper and its alloys after prolonged outdoor or indoor exposure is known as patina (Cano et al., 2005). Six copper surface treatments are considered here: mechanical polishing, indoor exposure for 7 days, chemical etching in 1.6 M nitric acid (HNO3), chemical etching and heating at 160 °C and chemical etching and dipping in 9 × 10−4 M or 0.9 M potassium sulphide (K2S) solution at 70 °C. As an example, Fig. 5.1 shows the reduction curves for mechanically polished copper specimens exposed for 7 days to the indoor atmosphere of the laboratory as a function of ν (potential scan rate, ν = dE/dt). Only one main broad ill-defined current density peak can be observed, which shifts to negative potential values as ν increases. Figure 5.2 shows the dependence of the current density peak (im) and the potential peak (Em) on ν for copper specimens subjected to the six tarnishing treatments. The expressions for im and Em describe, reasonably approximately, the linear relationship between im versus (ν)1/2 and Em versus (ν)1/2, derived from Müller’s model for film formation on electrodes (Müller, 1931). This can be described as follows: (i) im = (k1/k0)1/2(ν)1/2, where k0 is a constant (k0 = ρ/d), ρ is the resistivity and d the density of the tarnish film, k1 = nF/M, n is the charge on the metal cations, F is Faraday’s constant (96 500 C/mol) and M is the average molecular weight; and (ii) Em = (Rp + R0)(k1/k0)1/2(ν)1/2, where Rp is the resistance of the electrolyte solution inside the pores and R0 is the resistance of the external supporting test solution to the tarnish film. According to the equations for im and Em, there is a linear relationship between im, Em and (ν)1/2. The proportionality factor has the

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Current density (μA cm–2)

0

–500

0.44 mV s–1 1 5 10 20

–1000

–1500

–2000 –1.2

–1.1

–1.0 –0.9 –0.8 Potential (VSCE)

–0.7

–0.6

5.1 Cathodic stripping for mechanically polished copper specimens exposed for 7 days to the indoor atmosphere of the laboratory. A 0.1 M sodium acetate (NaCH3COO) pH 8 was used as supporting test solution.

dimensions of a resistance, and the film dissolution process may be said to be controlled by the ohmic resistance. The parameters im and Em increase linearly with the square root of ν, and the slope depends on the properties and thickness of the tarnish film. It should be noted that the (k1/k2)1/2 parameter can be used to estimate a ρ value of the tarnish film (Cano et al., 2005).

5.2.2 Electrochemical impedance spectroscopy (EIS) EIS is probably the most important electrochemical method used for the characterisation of coatings applied or formed on metal substrates, providing information about chemical reactions, corrosion, mass transport, adsorption–desorption processes and capacitance of the interfacial region. Impedance measurements have also been used to characterise such aspects of materials as their dielectric properties. The EIS method allows quantification of the three parameters defining a corrosion process: (i) the corrosion rate, through the charge transfer resistance (Rct) (Ω cm2) where Faraday’s law can be used to estimate the penetration of the attack (μm/year); (ii) the mass transport processes (diffusion) defined by the parameter (σ) (Ω cm2 s−1/2); and (iii) the electrochemical double layer capacitance at the metal/solution interface (Cdl) (F cm−2). By applying a low amplitude sine-wave voltage (V) signal across a test system ΔV = Vmsin(ωt), where Vm is intermediate voltage, it is possible to measure the frequency (f), f = ω/2π where ω is the angular frequency and

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Peak current density (mA cm–2)

(a)

0.0

0 –2

–0.5

–4 –1.0

–6 –8

–1.5

–10 –2.0 –12 –2.5

Peak potential (VSCE)

(b)

–0.05

Peak current density (mA cm–2)

134

–14 Polished 7 days lab. HNO3 160 °C 9×10–4 M K2S 0.9 M K2S

–0.10

–0.15

–0.20

1 2 3 4 5 Root of sweep rate (mV s–1)1/2

5.2 Dependence of (a) current density peak (im), and (b) potential peak (Em) on potential scan rate (ν) for copper specimens under different tarnishing treatments.

t the time. The phase shift or angle (φ) and the amplitude of the resulting sine wave current density (I) is given by ΔI = Imsin(ωt + φ), where Im is intermediate current density. Impedance (Z) is a vector defined as magnitude or modules (|Z| = Vm/Im and φ) containing both resistive (R) and reactive (C and/or L) components. Both of these types of components need to be determined in order to characterise the component. Z is represented in the complex plane as Z = Z′+jZ″ (Nyquist plot) where Z′ and Z″ are the real and imaginary parts respectively, and j = (−1)1/2. Another complex formalism is admittance (Y) which is defined as Y = 1/Z and, in the same way, Y = Y′ + jY″. The procedure for interpreting impedance measurements is to use a mathematical model or empirical equivalent circuit. The parame-

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ters can be estimated and compared with the experimental data (Bastidas et al., 2010). For analysis of the impedance data, a complex non linear least squares (CNLLS) procedure is frequently used: n

{

min ∑ [Zexp ′ (ω i ) − Zsim ′ (ω i )] + [Zexp ′′ (ω i ) − Zsim ′′ (ω i )] i =1

2

2

}

where Z′exp and Z′sim are the real, experimental and simulated impedance data respectively, Z″exp and Z″sim represent the imaginary impedance, n is the number of data points and ωi is the i-th angular frequency data point. As an example, Fig. 5.3 presents the Nyquist plot for an American Iron and Steel Institute (AISI) 316L stainless steel (SS) immersed for 30 minutes in a 5% sodium chloride (NaCl) solution and polarised at the pitting potential region (at 0.5 V vs saturated calomel electrode, SCE). The shape of the Nyquist plot shows a capacitive behaviour including: (i) a depressed semicircle at high frequencies (from 10 kHz to ∼251.19 Hz) with the centre lying below the real axis, which is associated with the frequency dispersion of impedance data; (ii) a capacitive loop at intermediate frequencies (∼6.3 Hz), which resembles an inductive-type loop over the real axis; and (iii) a third capacitive loop drawing a straight line or a portion of a second semicircle at low frequencies (∼<2.51 Hz). The simulation-fitting of the data in Fig. 5.3 was performed using the circuit depicted in Fig. 5.4, which was obtained using a CNLS fitting program. It should be said that the AISI 316L SS/NaCl

300

–Z'' (Ω cm2)

Experimental + Simulated 200

100

+

+

+ + + + + +

+

0.55 Hz + + + ++ + +++

+

200

100

+

300

Z' (Ω cm2)

5.3 Nyquist plot for an AISI 316L stainless steel in a 5% NaCl solution at 0.5 V vs SCE in the pitting region, after polarisation using a scan rate (ν) of 0.1 mV/s.

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Nanocoatings and ultra-thin films CPE Rs

C1

C2

R1

R2

Rct

Zw

5.4 Electrical equivalent circuit used to model the impedance data of Fig. 5.3.

Table 5.1 Parameters used in the simulation of impedance data of Fig. 5.3. Yp is a real frequency-independent constant (YCPE = Yp(jω)α), the dimensionless fractional α exponent (−1 ≤ α ≤ 1) is related to the width of distribution of relaxation time. α YP (μF RS (Ω cm2) cm−2 s−(1−α)) 5.32

56.36

σW C1 R1 C2 R2 RP (Ω cm2) (Ω cm2 s−1/2) (μF cm−2) (Ω cm2) (μF cm−2) (Ω cm2)

0.887 55.97

65.73

−1428

−18.30

267.02

−9.57

system can be considered a pseudo-time-invariant model in the frequency range 10 kHz to 0.63 Hz. The Warburg coefficient, see Table 5.1, is of the same order as RCT, possibly indicating ionic diffusion through the solid corrosion products precipitated near the pit mouth or in the pits, which can influence the diffusion process. Passivating species such as (MOOH)ads and (MOMOH)ads, which are related by the place-exchange mechanism and passivation film grown, may be responsible for the relaxation process shown by the negative values of R1 and C1 (Fig. 5.4). The formation of chlorocomplexes, for example (MOMOHCl)ads and (MOMCl)ads, may accelerate metal dissolution (M2+)sol using the following process (Bastidas et al., 2001; Polo et al., 2002; Bastidas, 2007): (MOMOH)ads + Cl− → (MOMOHCl)ads + e−

[5.1]

(MOMOH)ads + Cl− → (MOMCl)ads + OH−

[5.2]

(MOMCl)ads + H2O → (MOH)ads +(MOH)+sol + Cl−

[5.3]

(MOH)

+ sol

+

+ H → (M )sol + H2O 2+

[5.4]

In Eqs 5.1–5.4, species (MOMOHCl)ads and (MOMCl)ads are responsible for the relaxation process given by −R2, +C2 (see Fig. 5.4). Components R1, C1 and R2, which have negative values, do not physically correspond to passive electrical components but to an equivalent mathematical representation of the impedance (Polo et al., 2002). Negative pseudo-capacitance occurs when the surface coverage by a species decreases. This negative

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Real impedance (Ω cm2)

resistance results from interplay between the adsorption and electrodissolution processes. The AISI 316L SS/NaCl system should be unstable. In corrosion studies, the Kramers–Kronig (KK) test of impedance data is a highly valuable tool for data validation. Several factors may be responsible for an incorrect fit: systematic errors in the data file due to a change in the electrochemical system; or the use of an inappropriate equivalent circuit. The responsible factor can be distinguished using the KK relationships. Figure 5.5 shows KK-transformed data obtained using KK relationships from commercial copper in a 0.5 M citric acid (C6H8O7) solution, at 35 °C after immersion for 96 hours. The consistency of the experimental data is high, satisfying the KK relationships. The poisoning effect of the volatile chromium species, notably chromium oxy-hydroxide (CrO2(OH)2), on the electrode/electrolyte interfaces causes an increase in both diffusion and charge transfer resistance at the interface,

-Imaginary impedance (Ω cm2)

(a)

(b)

1000 800 600 400 200 0 10–3

+

Experimental + Simulated + + + + + ++ ++ ++ ++ + ++ ++ +++ +++++ +++++ ++++++ –2 –1 0 1 10 10 10 10 102 103 104 105 106

600 + 500 + 400 300 200 100

+ + + + + ++ ++++++++++ ++ +++ ++++++++++++++++

0 10–3 10–2 10–1 100

101

102

103

104

105

106

Frequency (Hz)

5.5 Comparison of (+) experimental data and (o) impedance calculated using KK relationships for copper in a 0.5 M citric acid. (a) Real impedance, (b) imaginary impedance.

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meaning that cell performance drastically deteriorates (Matsuzaki and Yasuda, 2001). Therefore, the alloy surface may have to be modified in order to improve stability. It has been reported that the application of conductive oxide coatings (thin films and nanolayers) not only decreases the growth rate of Cr2O3 scale, but also inhibits or even prevents the evaporation of chromium species from the Cr2O3 scale, which keeps the power output degradation rate to an acceptable level (Hilpert et al., 1996). Ideally, the conductive oxide thin film should be an electronic conductor with very low ionic conductivity, in order to minimise cationic and anionic ion transport through the coating. It is also preferable for the thin film to be dense and chemically compatible with adjacent materials. For instance, thermalsprayed coatings have led to stable long-term performance lasting for about 12 000 hours with a cell voltage degradation rate of less than 1%/1000 per hour. Furthermore, recent studies show that the electrical conductivity of the Cr2O3 oxide on ferritic SSs can be improved by dipping in NiO, lanthanum/titanium and magnesium (Haanappel et al., 2005). In the past, high temperature thin films were generally selected after the component design was finalised. Current designs require the substrate (typically a nickel-based superalloy) to have sufficient inherent resistance to degradation mechanisms to prevent catastrophic reductions in service lifetime in the event of thin film failure. Since the materials considered for future substrates may possess less environmental resistance at higher temperatures, the importance of thin films in achieving the required performance will continue to grow. In future, solid oxide fuel cell (SOFC) interconnect designs, coatings will be increasingly viewed as an integral portion of the design process to meet the high demands for system performance. Although many types of high temperature coatings are currently in use, they generally fall into one of three types: aluminide, chromide and MCrAlY. Thermal barrier coatings (TBCs), the family of coatings that insulate the substrate from the heat of the gas path, are becoming increasingly important as the number of performance benefits derived from their use increases. TBCs are ceramic coatings (partially stabilised zirconia) that are applied to an oxidation-resistant bondcoat, typically a MCrAlY or aluminide. The main problem with TBCs is that the abrupt change in composition and properties at the interface tends to promote ceramic layer spallation. The issues of initial cost, lifetime and reparability are of great importance for the components resulting from these developments, and as yet not all of these potential problems have been solved. Furthermore, there is currently insufficient experience in the field to identify the important issues and whether any of these relate to high temperature oxidation and corrosion. It is obvious that the growth of thermally grown oxide (TGO) on what is called the ‘bond coat’ is a matter of importance. The adherence between this and TBC oxide is also an interesting issue (Bastidas, 2006).

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The electrochemical behaviour of passive films on stainless steels can be interpreted in terms of the semiconducting properties of the films (Hakiki et al., 1995), and relationships have been formulated between the type of conductivity and the protective efficiency of the passive layers (Clerc and Landott, 1988). Capacitance measurements have often been employed in the study of passive films on several metals and alloys and the concepts of the electrochemistry of semiconductors have been extended to these systems. Capacitance studies are useful for obtaining information on the electronic structure of a semiconductor and, by the analysis of the Mott– Schottky plots (C −2 vs the applied potential U), it is possible to obtain the flat band potential and the donor concentration (Sánchez et al., 2007). The presence of a semiconducting oxide film determines the charge distribution and the potential drop at the metal–metal oxide–electrolyte interface, i.e. the character of the double layer. Capacitive studies of this system can yield significant information, not only about the film formation process but also about the electronic structure of the film. These films are most frequently non-stoichiometric oxides of amorphous or polycrystalline structure. The principles of the band theory of solids have been quite successfully extended to this type of materials (Hakiki et al., 1995).

5.2.3 Water absorption in organic coatings The coating capacitance (C) is proportional to the material dielectric constant (ε) and the specimen area (A) and inversely proportional to the coating thickness (t) (Murray, 1997; Cano et al., 2010): C=

εε o A t

[5.5]

where ε° is the permitivity constant for a vacuum (ε° = 8.85 × 10−14 F/cm). The C is a function of the applied frequency: C=

ΔI ΔEf 2π

[5.6]

where ΔI is the complex sinusoidal current measurement of the impedance data, ΔE is the complex sinusoidal voltage, f is the applied frequency and 2π is the usual conversion constant (Murray, 1997). One consequence of the relationships shown in Eq. 5.6 is that, as the measurement frequency is decreased, the sensitivity of the instruments must be increased proportionally. The C value in Eq. 5.6 is commonly used to evaluate the changes in the dielectric constant caused by water absorption or changes in the pigment/ polymer proportions of the protective coatings. The volume percentage of

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absorbed water (v/o H2O) generally follows the Brasher–Kingsbury empirical relationship (Brasher and Kingsbury, 1954): ⎡ log (Ct / Ct = 0 ) ⎤ v / oH 2 O = 100 ⎢ ⎥ log 80 ⎣ ⎦

[5.7]

where Ct represents the capacitance values at time t, or initially at t = 0 when no moisture was absorbed. In utilising Eq. 5.7, it is normally assumed that the polymer thickness remains constant.

5.2.4 Porosity of organic coatings The EIS technique can be used to evaluate the porosity of organic coatings. If the coating is of good quality, no significant changes occur on the specimen surface before measuring the anodic polarisation curves. In contrast, very porous films (of porosity greater than 1%) will already have failed after measurements of the corrosion potential (Matthes et al., 1991; Ahn et al., 2004). Determining porosity is difficult because of the small size of the defects. By using electrochemical measurements, porosity can be estimated from the electrochemical values. Assuming that the coating is electrochemically inert at low anodic overpotentials, Matthes et al. established an empirical equation to estimate the porosity (F) of the coating (Matthes et al., 1991): F=

Rpm (substrate) Rp( coating/substrate)

× 10 − ΔEcorr / βa

[5.8]

where F is the total coating porosity, Rpm(substrate) the polarisation resistance of the base metal, Rp(coating/substrate) the measured polarisation resistance of the coating, ΔEcorr the difference of corrosion potential between coating and substrate and βa the anodic Tafel slope of the base material.

5.3

Surface-sensitive analytical methods for ultra-thin film coatings

Surface engineering techniques can be used to develop a wide range of functional properties, including physical, chemical, electrical, electronic, magnetic, mechanical, wear-resistant and corrosion-resistant properties on the required substrate surfaces. Almost all materials, including metals, ceramics, polymers, and composites, can be used as coatings for similar or dissimilar materials. It is also possible to form coatings of newer materials (e.g., met glass, β-C3N4), graded deposits, multicomponent deposits, etc.

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5.3.1 Characterisation of ultra-thin films by atomic force microscopy (AFM) Atomic force microscopy (AFM) is a powerful imaging technique that, by scanning a sharp tip (typical end diameter 5–10 nm) over a surface, can produce topographical images which quantify surface morphology on an area scale comparable to that encountered by a colloid interacting with that surface (Binnig et al., 1986). In addition, AFM can directly measure the force of adhesion of a single particle in a direction normal to the surface at which the interaction is taking place. This latter technique involves the immobilisation of a single particle at the end of a tipless AFM cantilever, creating a ‘colloid probe’ (Ducker et al., 1992). This measurement can be made in either air or liquid, depending on which is most suitable for the process. Therefore, AFM is uniquely able to investigate both surface roughness and its influence on small particle adhesion. The colloid probe technique has been mostly used to quantify the forces acting during the approach of particles to surfaces (Butt et al., 1995), for example, electrical double layer interactions. There have also been studies of adhesion of hard inorganic particles to surfaces (Bowen et al., 1999), and of deformable particles to surfaces (Biggs and Spinks, 1998). The influence of particle surface roughness has been studied by measuring the adhesion of colloid probes made of different materials to atomically flat surfaces in a nitrogen atmosphere (Schaefer et al., 1995). However, the planar surfaces on which adhesion has been measured have mostly been relatively smooth. Theoretically, when the topographical peaks and troughs are of similar dimensions to the particle, the adhesion is increased or decreased relative to that measured on a flat surface, depending on where the interaction occurs. AFM allows measurement of the force between the colloid probe and the metal surface as a function of separation, where the separation is varied using a piezocrystal (Bowen et al., 2001). A laser beam reflected from the back of the cantilever falls onto a position-sensitive photodiode that detects small changes in the cantilever deflection. To other surfaces the scratches are less defined and have rough edges, leading to an overall granular appearance at this level. Mean surface roughness (Ra) may be calculated from the digitally data (BSI, 1988). This is defined as the arithmetic mean of the deviations in height from the image area mean – value (Z ), Ra =

1 n ∑ Zi × Z n i =1

where Zi is the height of each data.

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5.3.2 XPS study of corrosion protection of ultra-thin films With the development of modern surface analytical techniques, the understanding of the composition and structure of the surface of metals and alloys has been advanced to some extent. The corrosion resistance of stainless steels depends not only on the chemical composition and structure of passive films, but also on the chemical heterogeneity of the material. Much of the information about the composition and structure of SS passive film formed under different conditions has been obtained using XPS and AES. An abundance of chromium trioxide (CrO3) existed in the outer layer of passive film formed on the SS by electrochemical modification. As with the coexistence of chromic oxide (Cr2O3) and CrO3 in the barrier layer, the oxides of Fe2+ and Fe3+ species at the outer layer played a beneficial role in the formation of non-crystalline passive film. The XPS method allows quantitative and qualitative analysis from overall elements, except for hydrogen. In the case of iron, the presence of carbon and oxygen can be detected. This can be explained because at room temperature, metal surfaces which are in contact with the atmosphere are covered by a thin film formed by C-C/C-H, (OH)− and H2O (with a thickness lower than 3 nm). The C, O and Fe atomic ratio present on the metal surface can be calculated from the peak intensity (height). In this way, the existence of different iron oxide species such as Fe3O4, Fe2O3 and FeOOH can be deduced from the O/Fe atomic ratio (Pardo et al., 2007). The binding energy of the peaks associated with photoelectron emission is well defined, thus enabling the identification of the oxidation state of the cations and anions. Therefore, non-equivalent atoms belonging to the same element can be distinguished, marking the origin of a notable binding energy change (between 1 and 3 eV). This is known as ‘chemical shift’. (The non-equivalence of the atoms arises from differences in the oxidation state, the chemical surroundings or the crystalline network position.) On a high resolution O 1s spectrum for an iron surface, the component with high intensity appears at a binding energy of 530 eV, as a result of the presence of oxygen in the form of an iron oxide, and another two peaks with less intensity at 531.8 and 533.5 eV, which can be attributed to the presence of (OH)− and H2O, respectively. For the case of high resolution Fe 2p3/2, an XPS spectrum was recorded for an iron surface. The spectrum reveals two high intensity components at 709.7 and 711.0 eV, which can be associated to the presence of iron as Fe2+ and Fe3+, respectively. Considering each component area utilised during the O 1s y Fe 2p peak fitting and the O and Fe atomic ratio, the oxygen atomic ratio can be obtained, forming the oxides (OH)− and H2O. The Fe ratio in the form of Fe2+ and Fe3+ can also be obtained (Feliu Jr and Bartolomé, 2007).

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The XPS technique can reach a penetration depth analysis of 3 nm. This penetration depth allows the study of thin films and nanolayers, which encompasses the passivation layer of some metal and corrosion layers. Passivation processes of stainless steel can be studied using XPS. Olefjord found that using XPS, the presence of two layers could be determined: an inner layer mainly consisting of a Cr3+ oxide, whose thickness was dependent on the polarisation potential; and an outer layer of Cr(OH)3, whose thickness did not depend on the polarisation potential (Olefjord and Wegrelius, 1990). The outermost layer was found to include molybdenum in the form of Mo6+. This cation stabilises the passive oxide layer and protects it against chlorination. Compositional depth profile analysis can be performed by either nondestructive methods (when the thickness is between 5 and 10 nm) or destructive methods (when the thickness exceeds 30 nm). The non-destructive methods use (i) variation of the reflexion angle λ, so that at 90° information can be obtained for a depth of 3 λ while at 15° the analysis is focused on the outermost layer of the surface; (ii) comparison between two different levels of energy peaks; (iii) different x-ray energy sources (Mg, Al, Ag) to compare the kinetic energy for the same element. Regarding the destructive methods, the most common are inert gas (argon) ion-sputtering and ball-scattering. SnO2 thin films have attracted considerable interest due to their unique characteristics of high conductivity, high optical transmittance over the visible spectral region, excellent adhesion to glass substrates, chemical and thermal stability. These peculiar properties have found many applications in antistatic coatings, electrodes for flat panel and liquid crystal displays and sensors (Chopra et al., 1983). Moreover, transparent tin oxide coatings prepared by spray pyrolysis have classically been used as a protective coating in glass industries to strengthen glassware for both returnable and nonreturnable foodstuff bottles and jars.

5.3.3 Specular reflectance infrared spectroscopy characterisation of ultra-thin films on metallic substrates Specular reflectance sampling in Fourier transform infrared (FTIR) represents a very important technique useful for measurement of thin films on reflective substrates, analysis of bulk materials and measurement of monomolecular layers on a substrate material. Often the specular reflectance technique provides a means of sample analysis with no sample preparation, keeping the sample intact for other measurements. Specular reflectance provides a non-destructive method for measuring thin coatings on selective,

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smooth substrates without sample preparation. It basically involves a mirror-like reflection and produces reflection measurements for a reflective material, or a reflection–absorption spectrum for the surface film on a reflective surface. Thin surface coatings in the range from nanometers to micrometers can be routinely examined with a grazing angle (typically 70°–85°) or 30° angle of incidence, respectively. For example, lubricant thickness on magnetic media or computer disks is conveniently measured using this technique. The diffuse reflectance technique is mainly used for acquiring IR spectra of powders and rough surface solids such as coal, paper and cloth. It can be used as an alternative to pressed-pellet or mull techniques. IR radiation is focused onto the surface of a solid sample in a cup and results in two types of reflections: specular reflectance, which directly reflects off the surface and has equal angles of incidence and reflectance, and diffuse reflectance, which penetrates into the sample then scatters in all directions. Special reflection accessories are designed to collect and refocus the resulting diffusely scattered light using large ellipsoidal mirrors, while minimising or eliminating the specular reflectance, which complicates and distorts the IR spectra (Silverstein et al., 1981; Weightman et al., 2005). The basics of the sampling technique involve measurement of the reflected energy from a sample surface at a given angle of incidence. The electromagnetic and physical phenomena that occur at and near the surface are dependent upon the angle of incidence of the illuminating beam, the refractive index and thickness of the sample and other sample and experimental conditions. A discussion of all of the physical parameters and considerations surrounding the specular reflectance sampling technique is beyond the scope of this overview. Types of specular reflectance experiments include: (i) reflection–absorption of relatively thin films on reflective substrates measured at a near-normal angle of incidence; (ii) specular reflectance measurements of relatively thick samples measured at a near-normal angle of incidence; and (iii) grazing angle reflection–absorption of ultra-thin films or monolayers deposited on surfaces measured at a high angle of incidence. In the case of a relatively thin film on a reflective substrate, the specular reflectance experiment may be thought of as similar to a ‘double-pass transmission’ measurement. The incident FTIR beam, represented by I0, illuminates the thin film of a given refractive index (n2) and at an angle of incidence (θ1). Some of the incident beam is reflected from the sample surface, represented by IR at the incident angle θ1, and is also known as the specular component. The rest of the beam is transmitted into the sample represented by IT at an angle of θ2, calculated from Snell’s law: n1 sin (θ1) = n2 sin (θ2)

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At the reflective substrate, the beam reflects back to the surface of the thin film. When the beam exits the thin film, it has geometrically passed through the film twice and is now represented as IA. Infrared energy is absorbed at characteristic wavelengths as this beam passes through the thin film and its spectrum is recorded. The specular reflectance spectra produced from relatively thin films on reflective substrates and measured at a nearnormal angle of incidence are typically of high quality and very similar to spectra obtained from a transmission measurement. This result is as expected, because the intensity of IA is high relative to that of the specular component, IR. For relatively thick samples, the specular reflectance experiment produces results which require additional considerations, as the specular component of the total reflected radiation is relatively high. Again, the incident FTIR beam represented by I0 illuminates the sample of a given refractive index, n2, at the angle of incidence θ1. Some of the incident beam is reflected from the sample surface, represented by IR at the incident angle θ1. Some of the incident beam is transmitted into the sample, represented by IT at an angle of θ2. As predicted using Fresnel equations, the percentage of reflected vs transmitted light increases as the angle of incidence of the illuminating beam increases. Furthermore, the refractive index and surface roughness of the specimen and the sample absorption coefficient at a given wavelength all contribute to the intensity of the reflected beam. At wavelengths where the sample exhibits a strong IR absorption, the reflectivity of the sample increases. The superposition of the extinction coefficient spectrum with the refractive index dispersion results in a spectrum with derivative shaped bands. This specular reflection spectrum can be transformed using a Kramers–Kronig (KK) conversion to a transmission-like spectrum. Another useful application of specular reflectance is the measurement of relatively thin films and monomolecular layers at the grazing angle of incidence. At high angles of incidence (between 60 and 85°) the electromagnetic field in the plane of the incident and the reflected radiation are greatly increased, relative to a near-normal angle of incidence. The perpendicular component of the electromagnetic field of the reflecting radiation is not enhanced.

5.4

Spectroscopic, microscopic and acoustic techniques for ultra-thin film coatings

5.4.1 Infrared, Raman and Mössbauer spectroscopies The infrared spectroscopy is caused by the absorption of photons with energy corresponding to the IR region (14.300 cm−1–10 cm−1) into a

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molecule, which generates a transition between vibrational levels in that molecule, within the electronic state which is that species (Faraldos and Goberna, 2002). Identification of a compound from a spectrum of this kind is a two-step process. The first step involves determining which functional groups are likely to be present by examining the group frequency region, which encompasses radiation from about 3600 cm−1 to approximately 1200 cm−1. The second step then involves a detailed comparison of the spectrum of the unknown with the spectra of pure compounds that contain all of the functional groups found in the first step. Here, the fingerprint region (from 1200–600 cm−1) is particularly useful because small differences in the structure and constitution of a molecule result in significant changes in the appearance and distribution of absorption peaks in this region (Skoog and West, 2004). Therefore, this technique is useful for characterising the composition of coatings. For example, numerous titanium alkoxides, such as tetraisopropyl orthotitanate (TIPT), are used in sol–gel chemistry in order to prepare inorganic titanium oxides in the form of thin films for different applications. Burgos and Langlet (1999) created a qualitative FTIR spectrum analysis which aimed to assign the main absorption bands observed in the IR spectrum of sol–gel derived TiO2. This study showed that isopropoxy groups were completely replaced by ethoxy groups both for titanium isopropoxide diluted in ethanol and for xerogel film resulting from titanium isopropoxide solution diluted in ethanol and stabilised with hydrochloric acid. The main IR behaviour related to this exchange reaction was the complete disappearance of a band located at 1000 cm−1 and related to C–O stretching vibration in TIPT. Other research (Poultney and Snell, 2008) has shown that the FTIR technique is capable of producing spectra of colloidal silica (bands at 1000 cm−1) and phosphate (bands at 730 cm−1), which are important constituents of the coating mix and are used to construct the cores that carry the magnetic flux in electrical machines such as motors, generators and transformers. Therefore, this technique can be used to determine the composition of this coating and to detect changes in mix composition, which will improve the magnetic properties of the steel. On the other hand, when radiation passes through a transparent medium, the species present scatter a fraction of the beam in all directions. The visible wavelength of a small fraction of the radiation scattered by certain molecules differs from that of the incident beam. Furthermore, the shifts in wavelength depend upon the chemical structure of the molecules responsible for the scattering. The radiation emitted is of three types: Stokes scattering, anti-Stokes scattering and Rayleigh scattering (Skoog and West, 2004).

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Raman studies are therefore likely to yield useful information about certain functional groups that may not be revealed by infrared spectra. Raman spectroscopy is an appropriate technique for use in studying metal surfaces, in particular coatings and corrosion products. For example, a series of physical vapour deposition (PVD) ceramic hard coatings (TiN, ZrN, TiAlN, TiZrN and TiCN) were deposited on steel substrates using the cathodic arc/unbalanced magnetron deposition technique (Constable et al., 1999). These coatings were characterised using Raman microscopy to elucidate the behaviour of the optic and acoustic phonon modes of the (cubic) crystalline lattices. Defect-induced first- (and second)-order spectra have been observed in the 200–300 regions and these have been assigned and correlated with coating composition. Scattering in this acoustic range is believed to be primarily determined by the vibration of the heavy Ti ions), whereas in the optic range 500–800 cm−1 it is determined by the vibration of the lighter N ions. The bands due to acoustic modes in the multicomponent coatings appear to be more intense than those of the binary coatings and the acoustic modes are much more intense than the optic modes. Finally, in Mössbauer absorption spectrometry, a solid sample is exposed to a beam of gamma radiation and a detector measures the intensity of the beam transmitted through the samples. Typically, there are three types of nuclear interactions that are observed: (i) isomer shift (or chemical shift) reflects the chemical bonding of the atoms and is related to the electron density at the nucleus; (ii) quadruple splitting reflects interaction between the nuclear quadruple and the surrounding electric field gradient; and (iii) hyperfine splitting is a result of interaction between the nucleus and any surrounding magnetic field (Vértes et al., 1979). Mössbauer spectrometry provides a significant amount of information about electron density, chemical states, electron configurations and magnetic states around resonant nuclei. Conversion electron and x-ray Mössbauer spectra (CEMS and XMS) have been developed and used for characterisation of thin stainless steel films and thick-coated steel (Nomura et al., 2004). The as-deposited films consisted of one magnetic phase. The magnetic phase was partially converted into austenite inside the films by heating at 500 °C. The surface oxide layers produced by heating in air were composed of Fe2O3 and Cr2O3. The uniform thickness of the coating was about 3 μm.

5.4.2 X-ray diffraction When an x-ray beam strikes a crystal surface at an angle, a portion of the beam is scattered by the layer of atoms at the surface (following Bragg’s law). The unscattered portion of the beam penetrates to the second layer of atoms where again a fraction is scattered, and the remainder passes on

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to the third layer. The cumulative effect of this scattering from the regularly spaced centres of the crystal is diffraction of the beam (Skoog and West, 2004). X-ray diffraction (XRD) also provides a convenient and practical means for the qualitative identification of crystalline compounds. The x-ray powder diffraction method is unique in that it is the only analytical method capable of providing qualitative and quantitative information about the compounds present in a solid sample. X-ray powder methods are based upon the fact that each crystalline substance has a unique XRD pattern. This technique is used for the study of, for example, the behaviour of hydroxyapatite coatings on titanium substrates using an alkoxide-based sol–gel route as a function of temperature (Gross et al., 1998). XRD has revealed that a period of 24 hours is necessary for complete combination of the calcium and phosphite ethoxides to produce a hydroxyapatite coating, and that a final firing temperature of 800 °C is required to remove most of the organic material and produce a thin homogeneous hydroxyapatite coating. Other microstructural and compositional investigations using energy dispersive x-ray spectroscopy (EDS) and XRD have been carried out on cross-sections of FeAl coatings obtained by high velocity oxy-fuel (HVOF) spraying with different oxygen–propylene rates (F1, F2 and F3) (Guilemany et al., 2006). FeAl intermetallics have been intensively studied as potential substitutes for high temperature superalloys in some applications as both bulk materials and coatings. XRD scans show that oxide content is higher in F1 coatings (parameter set 1: oxygen flow rate: 189 and carrier gas (air): 385) and indicate that F2 (parameter set 2: oxygen flow rate: 189 and carrier gas (air): 305) coatings have a larger intermetallic phase than F1 (one peak observed at lower angle values indicates contribution to both the FeAl and hercynite phases).

5.4.3 Ion scattering, Rutherford backscattering and secondary-ion mass spectroscopy The impact of an ion with a few hundred electron volts or more of kinetic energy on a solid surface causes a series of collisional processes and electronic excitations. Analysis of the energy spectra of backscattered ions shows that they can provide detailed information about the atomic masses on the surface and about their geometric arrangement (Vickerman and Gilmore, 2009). In ion scattering spectroscopy (ISS), primary ion energies of 0.5–5 keV are used with noble gas ions (He+, Ne+, Ar+) and alkaline ions (Li+, Na+, K+). Information is obtained from the topmost atomic layer and also, under certain circumstances, from the second or third layer. An ISS spectrum

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represents the number of backscattered ions in the sample with a particular angle based on its kinetic energy. Ponjée et al. (2002) report a surface study of thin spin-coated and solution-cast films of poly(3-hexyl-thiophene) (P3HT), which contains a low concentration of siloxane. The siloxane-P3HT was chosen as a model system to demonstrate the unique capability of low energy ion scattering (LEIS) to quantify the surface composition of mixed systems and to obtain information on the position and orientation of molecules present on the surfaces of organic materials. These surfaces are enriched with siloxane due to the intermolecular segregation of the siloxanes present in P3HT. Although the number of siloxane monomers in the bulk is only 2% of the number of P3HT monomers, the siloxane coverage fraction was found to depend on the preparation parameters such as spin speed and solution concentration, and ranges from 25–100%. Segregated siloxane molecules on P3HT prefer specific sites such as the hexyl side chains, so that the sulphur atoms are screened from the surface by the methyl groups. In Rutherford backscattering spectrometry (RBS), the primary ion energy ranges from about 100 keV (for H+) to several MeV (for He+ and heavier ions). The ion–target atom interaction can be described using the Coulomb potential from which the Rutherford backscattering cross-section is derived, which allows absolute quantification of the results (Vickerman and Gilmore, 2009). The RBS spectra contain steps whose high energy edges relate to backscattered ions through the surface atoms. The minor energy signal corresponds to backscattered ions by the same atomic species but at greater depths. This technique can be used to obtain information on the composition of a coating, based on its depth. Popovic et al. (2008) studied the effects of ion irradiation on the microstructural and electrical resistance changes in TiN films. The films were deposited on Si substrates by reactive ion sputtering and subsequently irradiated with argon ions. Structural analysis of the sample was performed using RBS. The extracted depth profiles show a nearly uniform TiN layer stoichiometry and 1–2 at% of Ar throughout the layer thickness. An increased Ti yield was registered towards the Si substrate, which corresponds to the thin buffer layer. RBS spectra taken from samples deposited at 150 °C and RBS spectra after ion implantation adds up to an extra 2 at% of Ar remain essentially the same. RBS analysis suggests that ion irradiation does not induce any redistribution of components or intermixing at the layer/substrate interface. Secondary-ion mass spectroscopy (SIMS) is carried out by bombarding the surface of the sample with an ion beam. The ion beam is formed in an ion gun in which the gaseous atoms of molecules are ionised by an electron impact source. The positive ions are then accelerated by applying a high

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potential. The impact of these primary ions causes the surface layer of atoms of the sample to be stripped (sputtered) off, largely as neutral atoms. A small fraction, however, forms positive (or negative) secondary ions that are drawn into a spectrometer for mass analysis. SIMS provides a general surface analysis and a depth profile (Skoog and West, 2004; Fajardo et al., 2010). Thin zirconium oxide coatings (140–160 nm) were deposited onto iron samples using acetylacetone (acac) and hydroxypropyl cellulose (HPC) as stabilising agents. After the heating procedure, SIMS was used to characterise the coatings (Ugas-Carrión et al., 2010). Films prepared with acac showed two layers: a ZrO2 layer on the surface (0–500 s) and a deeper mixed oxide layer (until 2500 s). However, in the case of HPC samples, the SIMS profiles show a difference in the film structure: an extra and thicker amorphous mixed layer with a high concentration of nanocrystalline ZrO2 particles immersed on the substrate surfaces.

5.4.4 Glow discharge optical emission spectroscopy A glow discharge takes place in a low pressure atmosphere of argon gas between a pair of electrodes maintained at a specific potential. The applied potential causes the argon gas to break down into positively charged argon ions and electrons. The electric field accelerates the argon ions to the cathode surface that contains the sample (Skoog and West, 2004). The atomic vapour produced in a glow discharge is made up of a mixture of atoms and ions. A fraction of the atomised species present in the vapour is in an excited state. Relaxation of the excited species produces a low intensity glow that can be used for optical emission measurements. This technique provides information about the concentration and depth of each element present, through the conversion from intensity to concentration and from sputtering times to depth profiles. Solá-Vázquez et al. have investigated the feasibility of using radiofrequency glow discharge plasma spectrometry coupled with optical emission spectrometry as a rapid and simple tool to directly analyse bromine-containing solid materials coated with flame-retardant commercial paints (Solá-Vázquez et al., 2007). Polymeric layers for calibration were made by mixing appropriate amounts of tetrabromobisphenol A, bisphenol A, phloroglucinol and diphenylmethane-4,4′-diisocynate in tetrahydrofuran. Detection of bromine was investigated both in the visible (at 470–48 nm) and in the near-infrared (at 827–24 nm) regions, using a charge-coupled device for detection. Discharge parameters affecting the emission intensity of bromine were optimised (in argon and helium as possible plasma gases) and the analytical performance characteristics were evaluated. The best detection limit (0.044% Br) was achieved measuring Br I 827.24 nm in a He discharge,

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using a forward power of 70 W and a pressure of 45 Torr. The linearity range extended up to 27% Br. Another application of glow discharge optical emission spectrometry is in determining the components of zinc–nickel alloy coatings (Tian et al., 2009). Zinc–nickel alloy coatings are among the alloys most widely used for protecting substrate, because of their corrosion resistance and good mechanical properties. The corrosion behaviour of the coatings has been related to the components of nickel. It has been found that as the thickness of a zinc–nickel alloy coating increases, the amount of zinc increases from beginning to end, but the value of nickel peaks and an enrichment layer of nickel is formed in the coating, which increased its corrosion resistance.

5.4.5 Scanning electron microscopy and transmission electron microscopy The surface of a solid sample is swept in a raster pattern with a finely focused beam of electrons or a suitable probe. A raster is a scanning pattern similar to that used in a cathode-ray tube, in which an electron beam is (i) swept across a surface in a straight line, (ii) returned to its starting position, and (iii) shifted downward by a standard increment (Skoog and West, 2004). This process is repeated until a desired area of the surface has been scanned. Scanning electron microscopy provides morphologic and topographic information about the surfaces of solids. Thin films of LaCO3 were synthesised on stainless steel substrate by a simple technique: the ‘polymerisable complex (PC) route’ based on the polyesterification reaction between citric acid and ethylene glycol (Popa and Calderón-Moreno, 2009). LaCO3 thin films were grown at 600 °C for 3 hours. The microstructure of the materials was then evaluated using scanning electron microscopy (SEM). The micrographs show a dense and homogeneous film, with a polycrystalline microstructure of equiaxed grains sized about 40 nm. The film thickness obtained from SEM measurements is 0.5 μm. Finally, the bigger surface features observed in SEM images must correspond to aggregates of several primary crystallites and not to single crystallite domains. Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen and the image is magnified. A TEM provides two different but complementary types of information. It is able to produce images from the very thin struc´ ), but it can also be used to obtain diffraction ture of materials (100–200 Å diagrams (Albella et al., 1993).

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Ti1−xAlxSiyN hard coatings with an (Al + Si)/Ti ratio varying from 0.5 to 1.63 were studied by TEM and nanoindentation to determine the influence of their microstructure on hardness and fracture behaviour after heat treatment (1 hour at 1000 °C under nitrogen) (Parlinska-Wojtan et al., 2004). The addition of aluminium and silicon into the TiN system efficiently reduced the coatings’ grain size and finally resulted in the formation of a nanocomposite. The growth of an oxide layer on the top of the coatings was observed as a result of the heat treatment. The oxide-coating interface depends on the grain size and shape of the initial coating. It was found that films rich in Ti ((Al + Si)/Ti < 0.5) have a columnar microstructure and are formed by solid solution fcc- Ti1−xAlxN. When the (Al/Si)/Ti atomic ratio reaches about 0.8 a phase separation occurs, forming two cubic phases: Ti1−xAlxN and zinc-blende AlN. Finally, when the (Al/Si)/Ti ratio reaches 1.3, a hexagonal wurzite-type AlN phase also begins to form.

5.4.6 Scanning acoustic microscopy and Kelvin probe force microscopy Scanning acoustic microscopy (SAM) uses an acoustic signal, usually between 1 and 1000 MHz, transmitted through an aqueous ultrasonic coupling medium. The reflected signal is used to form an image of the sample, with contrast arising from differences in acoustic impedance (Briggs, 1992). SAM is a technique which can produce images showing variation in an object’s thermal and elastic properties, with a resolution of the order of microns. Scanning electron acoustic microscopy (SEAM) has been used to reveal the residual stress distributions in the sub-surface of carbon steel coating with Ti3N4 (Liao et al., 1999). Electron acoustic images (EAI) of carbon steel coating with Ti3N4 show residual stress distributions around cracks with a two-fold, symmetrical butterfly shape and a similar pattern of interchanging plastic and elastic zones. When the SEAM technique is applied to the natural surface of sintered piezoelectric transducer ceramics, the same butterflyshaped residual stress fields can be observed surrounding the pores of ceramics. The residual stress fields of the pores have the same orientation. Kelvin probe force microscopy (KPFM) is a non-contact, non-destructive method designed to measure the work function in a determinate point. This function can be defined as the minimum required work to remove an electron from Fermi level to the infinity (Cahn et al., 2000). This microscopy can be used to obtain both topographic and potential images. Potential images are obtained by detecting the cantilever deflection caused by an electrostatic force between the tip and the sample. This technique has been used to aid understanding of the protective mechanism of chromate conversion coatings (CCCs) on zinc. It has also been used to find alternatives to these coatings, because of health hazards

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invoked by chromium (VI) species (Zhang et al., 2005). The topography of the chromate coatings does not show any boundary between the chromate area and the bare zinc area. The Volta potential map shows that the potential in the chromate area is lower than that in the bare zinc area (about 80 mV). After immersion in 0.01 M NaCl solution for 24 hours, the topography of CCCs shows that the surface in the zinc area becomes rough and the potential difference between the chromate area and the zinc area becomes larger than before. This means that the zinc area was corroded and zinc oxide deposited on the surface. The chromate in the coating has a cathodic inhibitive effect on the corrosion of zinc.

5.5

Conclusions

Almost all types of material, including metals, ceramics, polymers and composites, can be deposited as coatings on similar or different materials. It is also possible to form coatings of newer materials (e.g., met glass, β-C3N4), graded deposits, multicomponent deposits, etc. These coatings can be characterised by electrochemical methods, and their surfaces can be studied using different analytical techniques such as AFM, XPS, infrared, Raman and Mössbauer spectroscopies, x-ray diffraction, ion spectroscopy, glow discharge optical emission spectroscopy (GDOES), electronic microscopy, SAM and KPFM. The dependence of the current density peak (im) and the potential peak (Em) on potential scan rate (ν) for copper specimens under tarnishing treatments can be described as a linear relationship between im, Em and (ν)1/2. The proportionality factor has the dimensions of a resistance, and the film dissolution process may be said to be under ohmic resistance control. The impedance method is a suitable tool for studying uniform and pitting corrosion of stainless steels, high temperature thin film corrosion in SOFC applications and general processes including charge transfer, diffusion and capacitance parameters.

5.6

Acknowledgements

The authors express their gratiude to Project BIA2008-05398 from the CICYT, Spain, for financial support. D. M. Bastidas gratefully acknowledges funding from Ramón and Cajal Program of the Spanish Ministry of Science and Innovation. M. Criado expresses her gratitude to the Spanish Research Council (CSIC) for her contract through the JAE Program co-financed by the European Social Fund.

5.7

References

Ahn S H, Lee J H, Kim J G and Han J G (2004), ‘Localized corrosion mechanism of the multilayered coatings related to growth defects’, Surf Coat Technol, 177– 178, 638–644.

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Albella J M, Cintas A M, Miranda T and Serratosa J M (1993), Introducción a la Ciencia de Materiales: Técnicas de preparación y caracterización, Madrid: Consejo Superior de Investigaciones Científicas. Bastidas D M (2006), ‘High temperature corrosion of metallic interconnects in solid oxide fuel cells’, Rev Metal Madrid, 42, 425–443. Bastidas D M (2007), ‘Interpretation of impedance data for porous electrodes and diffusion processes’, Corrosion, 63, 515–521. Bastidas D M, Cano E, Mora E M and Bastidas J M (2010), ‘Corrosion monitoring using impedance data’, in Makhlouf A S H (ed.), High Performance Coatings for Automotive and Aerospace Industries, New York: Nova Science Publishers, 351–384. Bastidas J M, López-Delgado A, López F A and Alonso M P (1997), ‘Characterization of artificially patinated layers on artistic bronze exposed to laboratory SO2 contamination’, J Mater Sci, 32, 129–133. Bastidas J M, Polo J L, Torres C L and Cano E (2001), ‘A study on the stability of AISI 316L stainless steel pitting corrosion through its transfer function’, Corros Sci, 43, 269–281. Biggs S and Spinks G (1998), ‘Atomic force microscopy investigation of the adhesion between a single polymer sphere and a flat surface’, J Adhes Sci Technol, 12, 461–478. Binnig G, Quate C F and Gerber C H (1986), ‘Atomic force microscope’, Phys Rev Lett, 56, 930–933. Bowen W R, Hilal N, Lovitt R W and Wright C J (1999), ‘An atomic force microscopy study of the adhesion of a silica sphere to a silica surface-effects of surface cleaning’, Colloid Surf A, 157, 117–125. Bowen W R, Lovitt R W and Wright C J (2001), ‘Atomic force microscope studies of stainless steel: Surface morphology and colloidal particle adhesion’, J Mater Sci, 36, 623–629. Brasher D M and Kingsbury A H (1954), ‘Electrical measurements in the study of immersed paint coatings on metal. 1. Comparison between capacitance and gravimetric methods of estimating water-uptake’, J Appl Chem, 4, 62–72. Briggs A (1992), Acoustic Microscopy, Oxford: Clarendon Press. BSI (1988), BS1134-1:1988 Assessment of surface texture. Methods and instrumentation, London: British Standards Institution. Burgos M and Langlet M (1999), ‘The sol–gel transformation of TIPT coatings: a FTIR study’, Thin Solid Films, 349, 19–23. Butt H J, Jaschke M and Ducker W (1995), ‘Measuring surface forces in aqueouselectrolyte solution with the atomic-force microscope’, Bioelectrochem Bioener, 38, 191–201. Cahn R W, Haasen P and Kramer E J (2000), Materials Science and Technology, Weinheim: Wiley-VCH. Cano E, Polo J L, La Iglesia A and Bastidas J M (2005), ‘Rate control for copper tarnishing’, Corros Sci, 47, 977–987. Cano E, Lafuente D and Bastidas D M (2010), ‘Use of EIS for the evaluation of the protective properties of coatings for metallic cultural heritage: a review’, Solid-State Electr, 14, 381–391. Chopra K L, Major S and Pandya D K (1983), ‘Transparent conductors. A status review’, Thin Solid Films, 102, 1–46.

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Clerc C and Landolt D (1988), ‘AC impedance study of anodic films on nickel in LiCl’, Electrochim Acta, 33, 859–871. Constable C P, Yarwood J and Münz W D (1999), ‘Raman microscopic studies of PVD hard coatings’, Surf Coat Technol, 116–119, 155–159. Ducker W A, Senden T J and Pashley R M (1992), ‘Measurement of forces in liquids using a force microscope’, Langmuir, 8, 1831–1836. Fajardo S, Bastidas D M, Ryan M P, Criado M, McPhail D S and Bastidas J M (2010), ‘Low-nickel stainless steel passive film in simulated concrete pore solution: A SIMS study’, Appl Surf Sci, 256, 6139–6143. Faraldos M and Goberna C (2002), Técnicas de análisis y caracterización de materiales, Madrid: Consejo Superior de Investigaciones Científicas. Feliu Jr S and Bartolomé M J (2007), ‘Influence of alloying elements and etching treatment on the passivating films formed on aluminium alloys’, Surf Interface Anal, 39, 304–316. Gross K A, Chai C S, Kannangara G S K, Ben-Nissan B and Hanley L (1998), ‘Thin hydroxyapatite coatings via sol–gel synthesis’, J Mater Sci-Mater M, 9, 839–843. Guilemany J M, Lima C R C, Cinca N and Miguel J R (2006), ‘Studies of Fe-40Al coatings obtained by high velocity oxy-fuel’, Surf Coat Technol, 201, 2072–2079. Haanappel V A C, Shemet V, Vinke I C, Gross M, Koppitz T, Menzler N H, Zahid M and Quadakkers W J (2005), ‘Evaluation of the suitability of various sealantalloy combinations under SOFC stack conditions’, J Mater Sci, 40, 1583–1592. Hakiki N H, Boudin S, Rondot B and Da Cunha Belo M (1995), ‘The electronic structure of passive films formed on stainless steels’, Corros Sci, 37, 1809–1822. Hilpert K, Das D, Miller M, Peck D H and Weiss R (1996), ‘Chromium vapour species over solid oxide fuel cell interconnect materials and their potential for degradation processes’, J Electrochem Soc, 143, 3642–3647. Liao J, Yang Y, Jiang X P, Hui S X and Yin Q R (1999), ‘Scanning electron acoustic imaging of residual stress distributions in ceramic coatings and sintered ceramics’, Mater Lett, 39, 335–338. Matthes B, Broszeit E, Aromaa J, Ronkainen H, Hannula S P, Leyland A and Matthews A (1991), ‘Corrosion performance of some titanium-based hard coatings’, Surf Coat Technol, 49, 489–495. Matsuzaki Y and Yasuda I (2001), ‘Dependence of SOFC cathode degradation by chromium-containing alloy on compositions of electrodes and electrolytes’, J Electrochem Soc, 148, A126–A131. Müller W J (1931), ‘On the passivity of metals’, Trans Faraday Soc, 27, 737–751. Murray J N (1997), ‘Electrochemical test methods for evaluation organic coatings on metals: an update. Part II: single test parameter measurements’, Prog Org Coat, 31, 255–264. Nomura K, Okudo T and Nakazawa M (2004), ‘Surface analysis of thin stainless steel films and thick-coated steel by simultaneous application of conversion electron and X-ray Mössbauer spectroscopy’, Spectrochim Acta B, 59, 1259–1264. Olefjord I and Wegrelius L (1990), ‘Surface analysis of passive state’, Corros Sci, 31, 89–98. Pardo A, Feliu Jr S, Merino M C, Arrabal R and Matykina E (2007), ‘The effect of cerium and lanthanum surface treatments on early stages of oxidation of A361 aluminium alloy at high temperature’, Appl Surf Sci, 254, 586–595.

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Parlinska-Wojtan M, Karimi A, Coddet O, Cselle T and Morstein M (2004), ‘Characterization of thermally treated TiAlSiN coatings by TEM and nanoindentation’, Surf Coat Technol, 188–189, 344–350. Polo J L, Cano E and Bastidas J M (2002), ‘An impedance study on the influence of molybdenum in stainless steel pitting corrosion’, J Electroanal Chem, 537, 183–187. Ponjée M W G, Reijme M A, Denier van der Gon A W, Brongersma H H and Langeveld-Voss B M W (2002), ‘Intermolecular segregation of siloxane in P3HT: surface quantification and molecular surface-structure’, Polymer, 43, 77–85. Popa M and Calderón-Moreno J M (2009), ‘Lanthanum cobaltite thin films on stainless steel’, Thin Solids Film, 517, 1530–1533. Popovic M, Stojanovic M, Perusko D, Novakovic M, Radovic I, Milinovic V, Timotijevic B, Mitric M and Milosavljevic M (2008), ‘Ion beam modification of structural and electrical properties of TiN thin films’, Nucl Instrum Meth B, 266, 2507–2510. Poultney D and Snell D (2008), ‘Use of the Fourier transform infrared (FTIR) technique for determination of the composition of final phosphate coatings on grain-oriented electrical steel’, J Magn Mater, 320, E649–E652. Sánchez M, Gregori J, Alonso C, García-Jareño J J, Takenouti H and Vicente F (2007), ‘Electrochemical impedance spectroscopy for studying passive layers on steel rebars immersed in alkaline solutions simulating concrete pores’, Electrochim Acta, 52, 7634–7641. Schaefer D M, Carpenter M, Gady B, Reifenberger R, Demejo L P and Rimai D S (1995), ‘Surface-roughness and its influence on particle adhesion using atomic force techniques’, J Adhes Sci Technol, 9, 1049–1062. Silverstein R M, Webster F X and Kiemle O (2005), ‘Infrared spectrometry’, Spectrometric Identification of Organic Compounds (7th edn), New York: Wiley, 72–126. Skoog D A and West D M (2004), Fundamentals of Analytical Chemistry, New York: Thomson-Brooks/Cole. Solá-Vázquez A, Martín A, Costa-Fernández J M, Ruiz Encinar J, Bordel N, Pereiro R and Sanz-Medel A (2007), ‘Quantification of bromine in flame-retardant coatings by radiofrequency glow discharge-optical emission spectrometry’, Anal Bioanal Chem, 389, 683–690. Tian W, Xie F Q, Wu X Q and Yang Z Z (2009), ‘Study on corrosion resistance of electroplating zinc-nickel alloy coatings’, Surf Interface Anal, 41, 251–254. Ugas-Carrión R, Sittner F, Yekehtaz M, Flege S, Brötz J and Ensinger W (2010), ‘Influence of stabilizing agents on structure and protection performance of zirconium oxide films’, Surf Coat Technol, 204, 2064–2067. Vértes A, Korecs L and Burger K (1979), Mössbauer Spectroscopy, Budapest: Elsevier. Vickerman J C and Gilmore I S (2009), Surface Analysis: the Principal Techniques, Chichester: Wiley. Weightman P, Martin D S, Cole R J and Farrell T (2005), ‘Reflection anisotropy spectroscopy’, Rep Prog Phy, 68, 1251–1257. Zhang X, Sloof W G, Hovestad A, van Westing E P M, Terryn H and de Wit J H W (2005), ‘Characterization of chromate conversion coatings on zinc using XPS and SKPFM’, Surf Coat Technol, 197, 168–176.

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6 Conventional and advanced coatings for industrial applications: an overview A. S. H. MAKHLOUF, Max Planck Institute of Colloids and Interfaces, Germany

Abstract: This chapter describes the role of coating technologies in some important industrial applications. The focus is on coatings for the automotive, electronics, and packaging industries. The chapter presents a critical review of recent research and developments on advanced coatings, such as smart coatings, ‘super’ hard coatings, multifunctional coatings, etc. Key words: nanocoatings, automotive industry, paints, aerospace coatings, smart coatings, sensors, packaging industry, bio-compatible coatings.

6.1

Introduction

This chapter reviews developments in the processes and types of coating used in particular industries, including: smart (e.g. self-healing) coatings, super-hard coatings, multifunctional coatings (e.g. in electronics), and coatings for particular purposes (e.g. thermal barrier coatings). It discusses the use of conventional and advanced coatings in the automotive and aerospace industries, in packaging (e.g. the food, pharmaceutical, and paper industries), the electronics sector, the paint industry, and in biomedical engineering.

6.2

Conventional coating technologies for the automotive and aerospace industries

Surface manufacturing technologies make a key contribution to safety and comfort in the automotive and aerospace industries. The following sections report past and present coating technologies in this field.

6.2.1 Chemical conversion coatings For most metals and alloys, corrosion reactions are slowed by a naturally occurring passive thin oxide layer formed at the surface in a specific environment. Artificial passivation films, which improve the corrosion resistance, 159 © Woodhead Publishing Limited, 2011

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have been produced by various processes. Because oxides are susceptible to mechanical damage, localized corrosion will occur at damaged sites. Aluminum alloys are widely used in the automotive and aerospace industries. Naturally occurring aluminum oxide, with associated flaws or defects, generally offers corrosion resistance for aluminum surfaces. However, for particular environments, or for decoration, an organic coating may be required. In order to enhance organic coating adhesion and durability, the aluminum is treated to ‘convert’ the original naturally occurring film to provide a tailored or functional conversion coating. Conversion coating can be carried out by free immersion or by electrical activation. The former involves direct immersion, spraying, rolling, etc., whilst the latter implies use of impressed currents. Moreover, the process involves immersion of the aluminum surface in a chemically reactive environment such as acid pickling or alkaline etching. Conversion coatings generally involve immersion processes wherein the naturally occurring aluminum oxide is transformed to develop coatings significantly thicker than 2.5 nm.1 Such coatings have applications in many areas, for example automotive, aerospace, packaging, etc. Other surface treatment processes – zincating, for example – could also be described as conversion coatings. Moreover, anodic electrocoating can also generate anodic alumina, sandwiched between the metal substrate and organic coating. The ideal conversion coating should be uniformly distributed over the substrate surface and have low environmental reactivity, good mechanical properties, and good adhesion. It should be environmentally acceptable, industrially applicable, and cost-effective. The most common conversion coatings are described in the following sections. Conversion coatings by hydrothermal treatment This coating system involves the immersion of aluminum in near-neutral water at elevated temperatures, which causes various forms of alumina to develop, including boehmite and bayerite. For particular capacitor applications, hydrothermal treatment of relatively pure aluminum foil is carried out in boiling water, to develop an outer region of pseudo-boehmite and an inner, less-well crystallized region of alumina material. The total film thickness is of the order of 300 nm. After hydrated film formation, the aluminum is anodized to form the dielectric material for application in capacitors.1 Chromate and chromate-free conversion coatings Chromate-based conversion coatings have received wide interest and have been employed extensively. Various films have been developed, including

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alumina and Cr2O3 in the presence of phosphate species in coating baths at 70 °C. However, due to the environmental and toxicity problems associated with the use of chromate conversion coatings, governments have insisted on the use of chrome-free processes, such as systems containing stannate, molybdate, zirconate, silicate, cerate, or titanate, etc. The coating system that is applied to aluminum surfaces for use in automobiles has different standard requirements to those for magnesium alloys. Since year 2000, a coating system with the following qualities has been developed for die-cast magnesium substrates: good paint compatibility, adhesion, corrosion resistance, and decorative appearance.2 The process involves applying a conversion coating (chromate, phosphate– permanganate, or fluorozirconate) to the surface, followed by a filler, and finally painting with a base coat then a clear coat. The filler used was a silicone-modified polyester resin, which improved the corrosion resistance and overall coating quality. However, the proposed process does not meet environmental requirements, because it is based on toxic chromates. Therefore, research continues into designing environmentallyacceptable coatings with high corrosion resistance and adhesion performance.

6.2.2 Organic and inorganic coatings Epoxy coatings and inorganic zinc-rich coatings have long been used to protect ships and marine structures from corrosion. Research and development in binder and pigments has improved the durability of coatings for other severe environments. The most common coating systems were silicone-modified polymers and fluorine-modified polymers. Moreover, two more new vinyl monomers have been developed for ambient curing coatings, namely: polysiloxane vinyl monomers with silanol groups, and alicyclic epoxy functional vinyl monomers.³ Automotive clear coatings, architectural paints, and some general industry coatings, are used because of their superior durability. Such coatings are highly durable and stain resistant, due to the resistance of their crosslinking bonds to water, chemicals, and ultraviolet radiation. The second technology that is commonly employed, F-NAD, uses fine dispersions of acrylic polymer in a fluoropolymer matrix prepared by dispersion polymerization. The dispersant is the copolymer of fluoroethylene and vinyl ethers (or vinyl esters), which has some hydroxy functionality and is soluble in various organic solvents. F-NAD is distinguished by a narrow particle size distribution as well as outstanding stability. Surface coatings formulated with this dispersion retain the fluoroplymer’s excellent weather and chemical resistance. Furthermore, high-solids dispersions, which form

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low VOC (300–400 g/L) surface coatings, were found to have the same features.3 Surface coatings formulated with F-NAD showed superior charcteristics, including: short curing time, good workability, high solids content, and excellent durability. F-NAD coatings were used for automotive applications, architectural paints, and some general industry purposes that require high performance coatings.2

6.2.3 Thermal-sprayed coatings Thermal spraying is a mature technology for forming hard coatings. Several processing routes can be employed depending on the nature of the substrate and the performance requirements of the coating, including: plasma spraying, high velocity oxy-fuel (HVOF) spraying, detonation flame spraying, and flame spraying. In plasma spraying, a high temperature plasma jet is developed inside the gun. Micro-particles powder is injected into a plasma jet to melt into droplets that are propelled towards the substrate.4 The HVOF method is based on a combustion process for heating and accelerating the powder coating material to high velocities. Combustion fuel gas such as acetylene, propane, propylene, or hydrogen is premixed with oxygen. The gaseous mixture is then ignited, causing a hypersonic flame with a velocity of approx. 2000 m/s. Powder particles are introduced into the combustion chamber by an inert gas carrier such as argon, then heated and accelerated towards the substrate under a hypersonic flow. High quality microcrystalline metallic and ceramic coatings can be fabricated by plasma spraying and HVOF spraying.4

6.3

Advanced coating technologies for the automotive and aerospace industries

Sustainability is an important topic for all manufacturers and scientists working in the transportation industry. Key goals include energy saving and reducing CO2 emissions by developing advanced technologies for high performance light-metal-alloy materials for fuel-efficient vehicles. Improving the mechanical properties, formability and corrosion resistance of aluminum and magnesium alloys has merited special attention. Another ongoing development is the improvement of automotive glazing by optimizing the optical and thermal properties of glass. Coatings with switchable transmission, and thin film solar cells as self-cleaning and selfhealing surfaces, will feature in the car of the future. The following sections report the most common current and future developmental approaches in that field.

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6.3.1 ‘Smart’ multifunctional coatings The development of a new generation of multifunctional coatings, with passive functionality but the capacity to actively respond to changes in their surroundings, is currently a hot technological topic. These coatings will provide key technology for the fabrication of future high-tech ‘smart’ coatings. The new multifunctional coatings will contain both passive and active components, which can provide fast responses as a result of changes occurring in the coating itself (such as scratches or cracks), or in the surrounding environment (such as temperature, pH, salinity). One of the important challenges in designing multifunctional ‘smart’ coatings is to develop nanoreservoirs (sometimes called nanocapsules or nanocontainers) that are compatible with the coating matrix, and which actively respond by loading and releasing the active material under external stimuli. Multifunctional materials also offer numerous possibilities for the development of components and devices that are lighter, stronger, stiffer, and more resistant to extreme environments. The nanocoatings and nanocomposite coatings developed in recent years are now not only able to sense corrosion and mechanical damage, but are also able to sense chemical and physical damage, promote adhesion and fatigue resistance, and also offer self-cleaning possibilities.5 Corrosion-resistant coatings with self-healing abilities are in demand for several industrial applications. Oxide–metal multilayer films, such as Al2O3/ Al/Al2O3/Al and Al2O3/Nb/Al2O3/Nb,6 were found to have self-healing abilities. However, corrosion proceeded preferentially along the metallic layer from intrinsic pinhole defects in the films. Oxide–metal nanocomposite films, such as Al2O3–Nb, exhibit excellent self-healing ability; but increasing Nb content in such films increases pinhole density within the film. A composition-gradient Al2O3–Nb nanocomposite film, with increased Nb content from the film surface to the substrate interface, was therefore developed, with an Al2O3/composition-gradient Al2O3-Nb/Nb/substrate configuration showing the best corrosion resistance.7 Self-assembling molecules are also a promising future category in surface coatings. Self-assembling molecules can arrange themselves on the substrate surface in an extraordinarily regular manner, and then polymerise to form a coating layer with self-healing ability. This very flexible selfhealing layer improves adhesion, corrosion protection, and mechanical strength.8 It is hoped that, in future, coatings will be designed with the ability to repair themselves after chemical or mechanical degradation. One approach to the development of these smart surfaces involves electroplated composite films containing capsules with corrosion inhibitors.

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6.3.2 ‘Super’-hard coatings Since year 2000, attention has increasingly been paid to ‘super’-hard ceramic coatings, due to their promising properties and many possible applications. These coating materials can be classified according to the character of their chemical bonds: metallic, covalent, or ionic hard materials. Metallic hard materials include the borides, carbides, and nitrides of the transition metals.9 ‘Super’-hard conventional coatings Nitride/carbide/boride coatings Various hard coatings, like nitrides, diamond and cBN, are currently used in the automotive industry, and are considered key to efficient automotive manufacturing processes. Several families of such coatings have been developed during the past 30 years, ranging from different metal nitrides, like TiN, TiAlN, or CrN, to various carbon-based films. Conventional nitride coatings have been prepared using physical and chemical vapor deposition. Superhard cBN will soon be available for industrial applications; this coating bridges the gap between nitrides and diamond, and has superior temperature stability to that of diamond. Due to their hardness and low friction coefficient, diamond-like carbon films will be indispensable for engine and power train components. However, their relevance to the automotive and aerospace industries is still limited, though it is likely to grow substantially in the future, pushed by the need to reduce fuel consumption and the desire for components with longer lifetimes.10 Nitride-based multilayered coatings, usually deposited by physical vapor deposition (PVD) techniques, have demonstrated improved durability for tribological applications. Multilayers can be divided into two categories: isostructural multilayers and non-isostructural multilayers. Isostructural multilayers contain individual layers that have the same crystallographic structure, such as TiN/NbN, TiN/VN, TiAlN/ZrN, and TiN/CrN. Nonisostructural multilayers consist of layers with different crystallographic structures, such as TiN/AlN, TiN/TaN, and TiN/CNx, providing a further barrier to dislocation motion.5 Moreover, superlattice multilayers exhibit significant hardness enhancement.11–15 The hardness range of most multilayered coatings is 30–60 GPa, and in general it will gradually decrease when temperatures increase to above 500 °C.16–18 Replacing heavy steel car bodies with hard, lightweight materials such as magnesium alloys is a major objective for automotive manufacturers. On the other hand, current research is also directed at improving corrosion protection of steel strips presently used for car bodies. Zinc magnesium alloys, or SiOx, deposited by plasma-activated evaporation, will most probably replace conventional corrosion protection in the near

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future. The combination of hardness and wear resistance in diamond-like carbon (DLC) films, and their piezoelectric properties, are the basis for the realization of novel temperature and force sensors. These can be used even in harsh environments and thus increase the comfort and safety of cars.10 Composite coating Despite innovations in thermal spray technology, which have led to almost negligible porosity within the coating microstructure, the reliability of thermal spray coatings can be compromised by the presence of tiny flaws like micro-cracks in the coating microstructure. This problem of flaws, surface imperfections, and micro-cracks can be overcome with a supplementary slurry coating, which seals and increases the density of the bulk coating. The slurry coating forms a thin, refractory-oxide layer, which bonds both internally and to the substrate with metal–oxide bonds.19 A composite coating system has been developed that consists of a substantial layer of hard metal from the tungsten carbide family, which is subsequently treated with a slurry coating to form a pore-free refractoryoxide layer.19 The slurry coating strengthens the interface between the hard metal coating and the operating environment, providing a robust physical barrier to highly corrosive constituents in its surroundings. ‘Super’-hard nanocoatings There is an increasing industrial demand for the development of high performance coatings with better oxidation resistance and hardness, and longer lifetimes than conventional nitride coatings. Therefore, significant effort has been devoted to the design and synthesis of super-hard coatings using nanotechnology. The availability of a variety of nanopowders has allowed researchers to develop nanocrystalline coatings. Fabrication of nanoparticles can be achieved via various processing routes based on vapors (such as PVD and chemical vapor deposition (CVD) to aerosol spraying), liquids (such as sol–gel and wet chemical methods), and solids (mechanical milling and mechanochemical synthesis). Different techniques have been used to fabricate nanocrystalline coatings, such as: thermal spraying, ion beam assisted deposition (IBAD), PVD, and plasma CVD. Conventional plasma spraying and HVOF thermal spraying have also been used to produce nanocrystalline coatings. Thermal spraying offers the unique advantage of a moderate to high rate of throughput, plus the ability to coat target materials with complex shapes using nanostructured powders. HVOF thermal spraying is very effective in depositing dense nanocrystalline coatings with superior

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wear properties. IBAD showed promising results as an effective technique for fabricating nanocrystalline coatings with good adhesion and controlled microstructures. This technique permits the deposition of nanocrystalline nitride films at lower temperatures, with better coating–substrate adhesion, and has fewer processing parameters than other PVD methods.4 However, the use of nanocrystalline coatings in general is still limited, because the synthesis of nanoparticles requires large-scale control that is unfeasible on an industrial scale. ‘Super’-hard oxide nanocoatings Thermal spraying methods that use nanopowders have yielded coatings with higher hardness, strength, and wear resistance than any produced by conventional coating methods. Their nanoscale microstructure can be retained despite the high temperatures and melting processes involved in forming a dense coating. Plasma spraying20–22 and HVOF spraying23,24 of nanostructured powders of metals or ceramics are effective techniques for forming nanocrystalline coatings. However, the HVOF method is preferred to plasma spraying because of its higher droplet velocity and lower thermal energy levels, which yield a denser structure, and higher bond strength between the coating and the substrate.4 Oxide-based ceramics such as alumina, chromia, titania, and zirconia have been widely used as surface coating materials to improve resistance to wear, erosion, cavitation, and corrosion. Zirconia-based coatings have been applied as thermal barrier coatings (TBC) for piston crowns and cylinder heads in internal combustion engines, in order to improve thermal efficiency, fuel economy, and power output.25 Nanostructured zirconia coatings with lower porosity exhibit a lower friction coefficient and high wearresistance compared to their microcrystalline coatings. ‘Super’-hard nitride nanocoatings Many attempts have been made to manufacture super-hard nitride nanocoatings. In particular, one should mention the considerable efforts that have been made to prepare nanocrystalline coatings with improved coating adhesion, using both the ion-assisted PVD methods and IBAD. Ion bombardment of the growing film can retard the grain growth and allow the formation of nanocrystalline films. IBAD shows great promise in forming metal nitride coatings because it significantly improves resistance to wear and corrosion. Furthermore, the desired electrical resistivity and optical properties are achievable with this method. The motivation for using the IBAD process comes from the need for independent control of the film composition and better adhesion of the film–substrate system.4 The devel-

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opment of hard nitrides nanocoatings such as TiN, CrN, VN, and ZrN using IBAD is at present the focus of intensive research. ‘Super’-hard nanocomposite coatings Recently, ultrananocrystalline diamond (UNCD) has been developed; a coating with good hardness and excellent wear resistance properties. UNCD possesses a hardness and modulus nearly equivalent to single crystal diamond. The friction coefficient of UNCD film is comparable with that of natural diamond, and wear damage on counter face materials is minimal owing to its smooth surface. Thus, UNCD and nanocomposite coatings are promising materials for use in the rubbing elements of microelectromechanical systems (MEMS).4 Previous data showed that super-hard nanocomposite coatings consisting of a nanocrystalline transition metal nitride embedded in a thin amorphous covalent nitride matrix exhibit hardness values >40 GPa. The main concept in the design of super-hard nanocomposite coatings involves preventing the formation of dislocations in the nanocrystalline phase, and blocking the grain–boundary sliding of nanograins.4

6.3.3 Self-cleaning coatings The disadvantageous optical and thermal properties of glass can cause discomfort for car drivers, and the glazed area in modern car designs has been increasing. Suitable transparent coatings that can limit these disadvantages are therefore increasingly in demand. Many efforts have been made during the last 20 years to replace mineral glass with polycarbonate (PC), mainly in order to reduce weight and improve safety. Coatings are needed to improve comfort by simplifying de-icing in winter and reducing heat transfer during summer.10 Highly scratch-resistant surface coatings are also needed to protect the soft PC, and an additional UV blocker has to be integrated in order to prevent degradation.10 To realize such complex coatings on an industrial scale, and at reasonable costs, is an ongoing challenge. On the other hand, it is expected that electrochromic coatings with variable transmission will replace mechanical blinds in sunroofs in the future, and they will also be used on side and rear windows for reasons of privacy. The traditional concept of ‘easy-to-clean’ glass will be replaced by a ‘selfcleaning’ concept, by integrating sensors or semi-transparent displays into the windscreen. Practically, many coatings that can fulfil the abovementioned criteria already exist, including highly selective sun control films with a transmission of more than 90% in the visible, and a high reflection in the near infrared, part of the solar spectrum. However, high

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manufacturing costs limit the use of such materials in automobiles. More research should be done in this field to make this technology industrially applicable.

6.3.4 Powder coating The main objective of powder coating technology is to provide coatings that are at least equal in performance to liquid coatings, without sacrificing ease of application or environmental advantages. Advances in powder coating technology aim at improving the materials’ durability and performance while using thinner films. Improvements in powder coatings tend to be aimed at providing costeffective replacements for liquid coating processes. Many efforts have been made towards improving coating quality. All of these improvements come at a cost, and often depend on other parts of the finishing process.26 As powder coating technology grows, and application equipment becomes more sophisticated, finishers need to understand the needs of their customers while meeting performance requirements and maintaining cost-effectiveness.

6.3.5 Transparent coatings Three types of transparent functional coatings have been produced by a modified physical vapor synthesis (PVS) process using nanocrystalline particles.27 Their applications include: transparent coatings to protect against UV and infrared radiation, transparent wear-resistant coatings, and conductive anti-static coatings. To form the radiation-resistant coatings, nano-sized ZnO, TiO2, antimony tin oxide (ATO), or indium tin oxide (ITO), are incorporated into commercial film polymers to form clear coats. ZnO and TiO2 absorb UV radiation and protect articles from UV exposure, while ATO and ITO absorb infrared radiation. Nano aluminum oxide was also incorporated into transparent clear coats to increase their hardness, scratch resistance, and wear resistance.Transparent coatings with nano-alumina offer improved abrasion resistance for transparent plastics, highly polished metals, wood, and laminate materials, without a deleterious effect on the surface appearance.27 Conventional photovoltaics uses a transparent layer of ITO to conduct energy. However, ITO film is expensive due to the costs associated with the vacuum fabrication method and the high value of indium. Recently, replacing the expensive ITO in solar cells with a lower-cost layer of graphene has been proposed as a solution. Graphene could be described as a sort of ‘atomic chicken wire’ crafted from bonded carbon atoms, and is applied to the photovoltaic via a spin-on process from solution.

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6.3.6 Thermal barrier coatings The improved performance of gas turbine engines is directly related to an increase in turbine inlet temperature; a result of improvements to the structural design and airfoil cooling, and use of high strength-at-temperature alloys as well as protective coating systems.28 Zirconia-based ceramics were considered for TBC applications due to their low thermal conductivities, high melting points, and thermal expansion coefficients.29–31 Zirconia–yttria TBCs are considered best at temperatures up to 1400 °C when deposited on a metallic bond coat.32,33 TBCs can be produced by thermal spray and electronbeam physical vapor deposition. However, the latter is more durable and thus is recommended for use on turbine blades and vanes in aircraft engines.34 In future, it will be necessary to develop TBCs that are not susceptible to the deposition of alkaline sulfate. These coatings will act as a barrier to the diffusion of alkaline sulfate, in order to protect the superalloy and ceramic matrix composites substrates from hot corrosion during aircraft engine operation. This research is already being undertaken by several engine companies.5

6.3.7 Modeling and computer simulations Theoretical modeling and computer simulations can provide details of atomic level structure and deformation that are not revealed by experimental methods. Large-scale molecular dynamics (MD) simulations have been carried out by several researchers, to provide a better understanding of the deformation behavior and mechanisms of grain boundary sliding, and of Coble creep diffusion of nanocrystalline materials. Tjong and Chen reported that this sliding is triggered by atomic shuffling and, to some extent, by stress-assisted free volume migration. However, molecular dynamics simulations also have certain limitations. MD simulations of the deformation of nanocrystalline materials are performed under high load and extremely high strain rate conditions. These impose restrictions on the number or the size of the grains that can be simulated. The simulated samples with a small number of grains may not represent the actual characteristics of real nanocrystalline materials. They also reported that deformation can only be initiated at the early stages, thus subsequent microstructural evolution and fracture damage resulting from prolonged deformation are excluded from the simulations. Nevertheless, MD simulations complement experimental techniques for exploring and explaining the deformation characteristics of nanocrystalline materials. The revolution in computer science – using advanced numerical methods and computer simulations – will no doubt be used increasingly for modeling microstructure and deformation characteristics of nanocrystalline materials. The unique nanoscale grain size is

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expected to exhibit novel properties, which will open new horizons for fresh technological applications.4

6.4

Packaging applications

6.4.1 Coatings for the food and pharmaceutical industries New approaches using nanotechnology-based thin films and coatings can be used to design, create, or model nanocoating systems with significantly optimized properties for the food, health, and biomedical industries. Topics for consideration include the employment of nanocomposite and nanostructured thin films in food packaging, security pharmaceutical labels, novel polymeric containers for food contact, medical instruments, bioimplants, and coated nanoparticles for bionanotechnology. Plastic containers are often used in the food and beverage industry, because they are lightweight, unbreakable, and resealable. PET bottles have almost replaced glass bottles and metal cans as the most common packaging for liquids such as carbonated soft drinks, tea, water, soy sauce, and edible oil. Nanocoatings, such as nanocomposite films, offer solutions to many challenges faced by the food packaging industry in relation to consumer health. For example, the unique properties of DLC, including its chemical inertness and impermeability, open up new applications in the food, beverage, and medical industries.35 However, other research should be done to reduce surface defects in DLC coatings, from which corrosion is usually initiated. Very recently, VTT Technical Research Centre of Finland developed novel, environmentally friendly, fully recyclable, thin, light, and air-tight packaging coating materials for advanced packaging solutions. This novel coating was developed using the atomic layer deposition (ALD) method; the coating has excellent gas permeation resistance and, as such, it is particularly suitable for food and pharmaceutical products. By using ALD coating, different functions can be integrated into the packaging material, such as properties that prevent water, oxygen, humidity, fats, and aromas from permeating the packaging, and protect the surface from stains and bacterial growth. ALD thus provides savings on raw materials and transport costs, because the amount of packaging material can be reduced. For example, chocolate wrappers can now be made without aluminum-coated paper, if the carton wrap is treated with the ALD coating method.36

6.4.2 Coatings in the paper industry Barrier-coated papers Barrier paper is produced by applying various types of coatings that impart resistance to permeation/penetration of certain types of permeates.37–39 A

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huge number of barrier-coated papers is produced today, from pizza boxes to burger wrappers to corrugated boards. Environmental issues, cost pressures, and dwindling profitability are forcing mills to rethink their coating strategies. Coating technology is one critical area that leads to radical changes in the barrier coating paradigm.40,41 Most of the current coating processes have some inherent disadvantages: pinholes, incomplete coverage (especially at low coat weights), base sheet penetration, saturation, and non-uniform coating layers.42–44 These limitations necessitate higher coat weights. Since barrier coatings are some of the most expensive paper coatings, any improvement would result in substantial savings.45 Curtain coating Curtain coating has been extensively used by the photographic paper industry for a number of decades, and is now emerging as a technology with great potential for the specialty coated paper industry. A curtain coater is a noncontact metering method, which provides excellent coverage and a uniform coating layer. The uniform thickness of the coating layer makes curtain coating particularly attractive to specialty paper markets such as barrier coating. Curtain coating is a pre-metered type of coating operation, with the coating layer being formed before it comes into contact with the substrate. Because a uniform pinhole-free liquid curtain is formed before it is exposed to the substrate, the integrity of the coating lattice and coverage is virtually guaranteed. Curtain coating seems to be most suitable for barrier applications because it delivers uniform, pinhole-free coating layers with high coverage at low coat weights.45

6.5

Coatings for the electronics and sensors industry

6.5.1 Sensors The extreme stresses and temperatures in combustors and turbines used in automobiles and aerospace vehicles lead to severe thermal and thermomechanical damage, which shortens the lives of engines. On the other hand, due to the application of protective coatings and advanced materials such as ceramic matrix composites (CMCs) in aircraft engines, traditional wire strain gauges and thermocouples cannot be attached to the components via conventional spot welding techniques. Therefore, reliable but non-intrusive sensors that can provide both temperature and strain information are urgently needed to measure and model such damage.46 Ceramic-based thermocouples are known for their stability at high temperatures. However, they are only available as rods or probes.47 The

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development of thin film ceramic-based thermocouples remains a topic for future research.48,49 Sensors in thin film form, embedded at the surface of components, need to be developed for measuring surface parameters. These thin film sensors should have a thickness of only a few microns, and thus add negligible mass to the surface, creating minimal disturbance to gas flow. Consequently, they should have minimal impact on the thermal, strain and vibration patterns that exist in the operating environment.46 Sensors based on electrically conductive ceramic materials, such as silicon carbide matrix composites, are thermally oxidized to form a stable, adherent silicon dioxide layer. Another layer of PVD-deposited alumina is then added in order to obtain the required insulation resistance. The sensors are fabricated onto the alumina layer, and then an alumina overcoat is deposited by PVD for protection.5 Sensors based on nickel–chrome and palladium–chrome thin films have been successfully deposited onto nickel-based superalloy substrates, and showed good strain sensitivity at high temperatures.50–52 Sensors based on Pt/Pd were also used as thin film thermocouples. However, they cannot provide stable output over 850 °C.53,54 Thermocouples based on ITO thin films showed promising results for high temperature applications up to 1400 °C.55,56 Static strain testing indicated that the ITO thin film strain gages survived repeated cyclic loading for tens of hours at temperatures up to 1581 °C.57 Sensors based on PtRh/Pt are popular thermocouples for temperature applications up to 1200 °C.58 However, they exhibit some problems at high temperatures, such as substrate reaction, rhodium oxidation, and film separation.59–61

6.5.2 Electronic nanodevices Advanced nanotechnology for the fabrication of electronic nanodevices and microelectromechanical systems (MEMS) requires the controlled assembly of well-ordered structures and the design of wear-resistant superhard nanomaterials. Research is currently in progress for the development of advanced nanodevices using a variety of new functional building blocks based on nanoparticles, nanowires, and nanotubes or their self-assemblies. Growing interest in designing nanodevices has motivated researchers to seek a better understanding of the structure–property relationship of nanocrystalline materials, in order to successfully modify and manipulate these materials at the nanoscale.4 One current challenge is finding ways to prepare high purity nanomaterials at relatively low cost. The instability of nanocrystalline materials at moderate or high temperatures is a second issue. The beneficial super-

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plastic properties achieved in bulk nanocrystalline materials at lower temperatures are offset by rapid growth of these nanograins during deformation. Thus, maintaining the stability of bulk nanocrystalline materials for structural applications is also a present difficulty.

6.5.3 Photovoltaic surfaces In terms of renewable energy sources, photovoltaic (PV) energy production is one of the most promising technologies. According to the EPIA, the cumulative installed capacity of solar PV systems around the world had reached more than 9200 MW in 2007.62 Cadmium telluride (CdTe) and copper indium disulphide/diselenide (CIS) thin films increasingly became the focus of research because they are easy to use and relatively inexpensive. Moreover, the estimated lifetime of these thin films is about 25–35 years of power generation. The rapid R&D and the sharp growth of the photovoltaic market will certainly provide solutions to future energy problems. However, several questions have recently been raised regarding the environmental problems associated with the end-of-life management of photovoltaic waste. Thin film-based PVs will make a big impact on energy in the future. However, when they reach their end-of-life, heavy metals are hazardous substances; toxic and carcinogenic to human beings, and harmful to the environment if they are not recycled or disposed of properly. Therefore, suitable cycling and processing methods must take these facts into account.63 Recent trends in VPs are moving towards designing thin film silicon photovoltaics for low-cost solar energy capture. The current market for solar energy has stalled due to the high cost and low efficiency of current technology. Conventional solar cells do not produce power efficiently enough to justify the cost of investment. Several international projects are currently seeking to develop low cost, thin film silicon photovoltaics with high efficiency, which could be used to realize the full potential of solar energy. One interesting approach is based on the optimization of thin film silicon photovoltaic research by exploring lower cost techniques for their manufacture, using layers of thin film silicon in an enhanced cell design.

6.6

Paints and enamels industry

Applying nanotechnology in the coatings field opened the door to new physical instruments, and to more advanced materials and coatings processing. The unique features of nanoscale coatings erased the barriers between chemistry, physics, and biology. Nanotechnology offered essential innovations for the development and formulation of industrial coatings and

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paints. The coatings synthesized by nanotechnology showed unique optical, mechanical, and chemical properties.8 Over the next decade, the coatings industry evolved by applying nanotechnology to the design of environment-friendly paints, powder coatings, and radiation-curing systems. Research in industry and academia was pushed in the direction of alternative technologies by new legislation relating to eco-friendly coating systems. In the same manner, there is a current trend towards the removal of heavy metals from enamels, and towards the production of solventless or solvent-free enamels. New monomer building blocks will be used in the future, and some of the monomers used today will be eliminated due to their toxicity. High quality binders with new chemistry and architecture will be designed. New binders with tailor-made properties and defined molecular structures will be used. These binders will play an important part in building up tailormade coating properties. Fortunately, recent studies have shown that new binding agents based on natural resources such as cellulose or chitin will make organic raw materials attractive as components for enamel. Hybrid-polymers of organic polymers and organically modified, inorganic silicates have great potential for future coatings systems. Such combinations offer special advantages, such as chemical stability and scratch resistance, due to the inorganic networks and high elasticity of the organic polymers.8 Hybrid polymers have also shown good adhesion to glass, which will provide new opportunities for functional glass deposition technology. Recently, a multinational report analyzed the percentage of the toxic lead element found in different paints, and revealed high levels in three countries (China, India, and Malaysia) and low levels in a fourth country (Singapore) where a relevant regulation was enforced. The average lead concentration ranged from 6988 (Singapore) to 31960 ppm (Ecuador). The report warned of the possible export of lead-painted consumer products to other countries, and the dangers the lead paint represented to children in the countries where it was available for purchase.64 There is an increasing demand for a worldwide ban on the use of lead in paints. An urgent need exists to establish effective worldwide controls to prevent the needless poisoning of millions of children from this preventable exposure.

6.7

Biomedical implants industry

Coatings for medical applications will also be the focus of future research, particularly in the areas of orthopedic implants and bone tissue engineering. Hydroxyapatite (HA) is a bioactive material, because its chemical structure is close to that of natural bone. Its bioactive properties make it an attractive material for use in biomedical applications. Titanium and some

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of its alloys, such as Ti–6Al–4V, are widely used as orthopaedic and dental implant materials due to their low elastic modulus, good biocompatibility, and corrosion durability. They are lightweight, with high strength to weight ratios. However, bone does not bond directly to these materials because they become encapsulated by fibrous tissue after implantation, which isolates them from the surrounding bone.65–67 HA coatings are promising because they can exploit the biocompatibility and bone bonding properties of the ceramic, while utilizing the mechanical properties of substrates such as Ti–6Al–4V and other biocompatible alloys. While the metallic materials have the required mechanical properties, they also benefit from the HA, which provides an osteophilic surface for bone to bond to, anchoring the implant and transferring load to the skeleton, helping to combat bone atrophy. It has been observed that a bioactive titanium surface can be prepared using a simple chemical treatment.67–70 In order to enhance bone-bonding ability, titanium alloys are often coated with HA by various methods.69–73 The beneficial biocompatible properties of HA are well documented. It is rapidly integrated into the human body, while at the same time the body is unaware of the presence of a foreign body. Perhaps its most interesting property is that HA will bond to bone, forming indistinguishable unions. However, poor mechanical properties (in particular fatigue) mean that HA cannot be used in bulk for load-bearing applications such as orthopaedics. Of the available coating techniques, thermal spraying is the most commonly used and analyzed method for HA coating. This technique has been faced with the challenge of producing a controllable resorption response in clinical situations. Thermally-sprayed coatings are being continually improved through the use of different compositions, and by post-heat treatments, which convert amorphous phases to crystalline calcium phosphates. Other techniques have also been investigated, such as pulsed-laser deposition and sputtering which, like thermal spraying, involve high temperature processing. Research projects have been proposed based on low temperature techniques such as electrodeposition and sol–gel, in order to avoid the challenge associated with the structural instability of HA at elevated temperatures. Table 6.1 summarizes some techniques that have been proposed with the advantages/disadvantages of each one.73,74

6.8

Acknowledgements

Many thanks to all the authors of papers, books, and websites and all published sources (listed in the references) that were used to prepare the materials of this chapter.

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Table 6.1 The most common techniques for hydroxyapatite coatings formation Technique

Thickness

Advantages

Disadvantages

Dip coating

0.05–0.5 mm

Requires high sintering temperatures, thermal expansion mismatch

Sputter coating

0.02–1 μm

Inexpensive, coatings applied quickly, can coat complex substrates Uniform coating thickness on flat substrates

Pulsed laser deposition Hot pressing and hot isostatic pressing

0.05–5 μm 0.2–2.0 mm

Electrophoretic deposition

0.1–2.0 mm

Thermal spraying

30–200 μm

Sol–gel

<1 μm

As for sputter coating Produces dense coatings

Uniform coating thickness, rapid deposition rates, can coat complex substrates High deposition rates

Can coat complex shapes, low processing temperatures, relatively cheap as coatings are very thin

Line of sight technique, expensive, timeconsuming, cannot coat complex substrates, produces amorphous coatings As for sputter coating HP cannot coat complex substrates, High temperature required, thermal expansion mismatch, elastic property differences, expensive, removal/ interaction of encapsulation material Difficult to produce crack-free coatings, requires high sintering temperatures

Line of sight technique, high temperatures induce decomposition, rapid cooling produces amorphous coatings Some processes require controlled atmosphere processing, expensive raw materials

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7 Nanocoatings for architectural glass J. MOHELNIKOVA, Brno University of Technology, Czech Republic

Abstract: Window glass is an important building material that influences thermal and visual comfort in buildings. Glazings with spectrally selective coatings are suitable for many architectural applications. Thin film selective coatings can be used to modulate the optical and thermal properties of glass, and low emissivity glazings reduce radiative heat loss and aid passive solar control. Special glazings use chromogenic materials for dynamic modulation of light and solar transmittance in response to external stimuli such as temperature, solar radiation or electric current. A combination of chromogenic glazings with light redirecting systems, enhanced transparency and special protective and self-cleaning surfaces complete the list of smart glazing devices. This chapter presents an overview of several types of window glazings and coatings. Key words: window glazings, glass coatings, low-emissivity, light transmittance, reflectance, solar radiation, daylighting, infrared radiation, wavelength.

7.1

Introduction

High-rise buildings with glazed curtain walls together with the drive to make them energy efficient have been largely responsible for development in the field of architectural glazings, and the main focus of window glass design has been on improving its optical and thermal properties.1,2 Special highly transparent and heat-reflective glazings can be used to save energy by reducing heat loss through windows, as well as using solar transmittance control to minimise cooling and air conditioning costs.1–3 Some window glazings are also used for light – enhancing and redirecting or decorative purposes. Double or triple glass panes sealed together as one glass product with distant cavities are used as window glass units in temperate climate.2,3 Radiative heat loss from a standard double-glazed, air filled unit is about double (about 65%) the heat loss through convection and conduction (about 35%).4 Therefore, to achieve higher thermal performance of windows, radiative heat loss needs to be reduced.2–4 Thin film coatings deposited on the surface of a pane of glass during the manufacturing process modulate its optical and thermal properties.1–4 The optical properties of thin films depend on the materials used and the layer 182 © Woodhead Publishing Limited, 2011

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composition in a coating as well as on the quality of the glass substrate surface and the coating deposition technology used.5,6 There are two main groups of coatings used for architectural applications, as spectrally selective coatings for standard windows and as chromogenic thin films for switchable glass devices. Glasses with spectrally selective coatings can be classified according to their influence on the indoor climate: as a glass with limited or enhanced light transmittance and as glazings with enhanced infrared reflectance.7 Special angular selective thin films are also used for solar control applications.2,8 The above-mentioned angular and spectrally selective glazings can be classified as passive, because their optical properties do not vary in response to external stimuli. Chromogenic glazings can dynamically modulate solar radiation transmittance, and are used in intelligent switchable solar control systems. The glazing include chromogenic materials which facilitate reversible colouring procedures triggered by physical or chemical stimuli.1,2,9–11 The chromogenic glazings can also be combined with solar holographic or microstructured transparent materials for smart glazing devices. Recently developed nanotechnology processes, focused on precise and premediated manipulation of atoms of materials, provide new possibilities for enhancing the physical properties, durability and self-cleaning effects of glazing systems.12,13

7.2

Spectrally selective glass

7.2.1 Optical properties and emissivity of glass The optical properties of glass5,6 are generally determined as spectral reflectance ρ(λ), transmittance τ(λ) and absorptance α(λ) in a specified wavelength λ. These properties interrelate in the following way, based on the principle of energy balance: ρ(λ) + τ(λ) + α(λ) = 1

[7.1]

When designing architectural glass, it is very important to take into account the solar radiation affecting a glass pane in the spectral range λ ∈ 〈300 nm;2500 nm〉14 and especially in the spectrum of visible light λ ∈ 〈380 nm;780 nm〉. Other important spectra include the near infrared solar range λ ∈ 〈780 nm;2500 nm〉 as well as the infrared (IR) radiation of building interior surfaces λ ∈ 〈3 μm;50 μm〉.15 Solar radiation transmitted through windows is partly absorbed by nontransparent interior surfaces. Solar radiation energy transforms into thermal energy once it has been absorbed. The building constructions are warmed and their surfaces emit IR radiation. Standard window glass is opaque for long-wavelength IR radiation16 and there is also very low glass reflectance

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in the IR range, meaning that if ρ(λ) → 0 and τ(λ) = 0, absorptance is increased α(λ) → 1 in the long-wavelength IR range. According to Wien’s law,17 if the temperature is 20 °C (room temperature) then the wavelength at which the radiation curve peaks is 10 μm. These temperature and wavelength values are important for the design of window glasses. Stefan–Boltzmann’s law17 states that radiation between surfaces depends on emissivities (ε) of surfaces. Energy M(λ,T) [Wm−2] emitted from a real grey body at temperature T [K] is, according to Kirchhoff’s law,17 derived as: M(λ,T) = α(λ)σT4 = ε(λ)σT4

[7.2]

where α(λ) is absorptance, ε(λ) is emissivity and σ is the Stefan–Boltzmann constant.17 It is clear from Eq. 7.2 that absorptance is directly proportional to emissivity in the IR range. As noted, standard window glass has very high IR absorptance. Practically, this means that standard window glass undergoes high heat radiation losses on cold days with low outdoor temperature, due to high glass emissivity. On the basis of the aforementioned assumptions, the relationship between reflectance and emissivity in the IR range is:18,19 ρ(λ) = 1 − ε(λ)

[7.3]

Heat loss through the glass is reduced due to a thin low emissivity coating deposited on its surface, preferably a coating with high visible transmittance for λ ∈ 〈380 nm;780 nm〉 and very high reflectance in spectral range λ ∈ 〈3 μm;50 μm〉. The higher the IR reflectance is, the lower the emissivity of the surface. Standard glass mean emissivity is ε = 0.8353,19 and the mean IR reflectance reaches a value of ρ = 0.165. Low-emissivity glazings have ε between 0.1 and 0.05 or less, which indicates a high reflectance of 0.90 to 0.95 or even higher.2,3,20

7.2.2 Glass with low-emissivity coatings The low-emissivity (low-e) glazings show spectral selectivity in high visible transmittance and high IR reflectance. Two types of coatings are used for the following practical applications:3, 4,20–37 •

•

low-e coatings for reduction of heat losses through the glazing due to reflection of long-wavelength IR radiation back into the building interior; low-e solar control coatings for the elimination of solar gains due to the reflection of the near IR part of solar radiation into the exterior.

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Doped oxide semiconductor thin films, consisting of dielectric layers with mobile charge carriers, can be used as low-e coatings. Materials such as SnO2:F, In2O3:Sn, SnO2:Sb, ZnO:Al or Cd2SnO4 are used.20,22–27 Microgrid conducting coatings also prove low emissivity but they are not commonly used for window glazings.20 IR reflective films of dielectric/metal/dielectric multilayers are widely used where low-e coatings are required.20–38 Metals such as silver, gold and copper have high IR reflectance21,28,29 and silver is frequently used because it provides a high IR-reflective and colour neutral coating.14,18,19 Gold-based reflective coating36,37 technology is very expensive for window glazings and is not colour neutral. However, new nanomaterials offer gold nanoparticle coatings with optimised reflectance and colour.38 Thin dielectric films protect the metal film and increase its light transmittance. Frequently used dielectric materials for the low-e coatings are TiO2, SnO2, SiO2, ZrO2, ZnS, ZnO, SnBO2, In2O3, Si3N4 and Bi2O3.20–39 Dielectric layers serve an anti-reflective purpose in coatings. Protective layers such as seed and sacrificial layers are also applied in low-e coatings.39 A seed layer undercoats the metal film and optimises crystalline growth of a thin layer of silver. A sub-oxidic interface layer on the silver film is also called the ‘sacrificial layer’. An example of a typical low-e coating composition with anti-reflective, metal, seed and sacrificial layers can be found in Table 7.1.39 The aforementioned low-e coating exists in two variations as: •

symmetrical coating: glass/SiO2/ZnO/Ag/TiOx/SnO2/SiNxOy with two anti-reflective layers of the same refractive indices; • asymmetrical coating: glass/TiO2/ZnO/Ag/TiOx/SnO2/SiNxOy with dielectric anti-reflective layers of different refractive indices: titanium dioxide with higher refraction index 2.5 and tin dioxide with lower refractive index 2.0 – this composition creates a low-e coating with higher light transmittance than the similar symmetrical coating.39

Table 7.1 Low-e coating layer Layer

Material

Function of a layer in the low-e coating

Protective coating

SiNxOy

Anti-reflective layer Interface layer Metal layer Seed layer

SnO2 TiOx Ag ZnO

Anti-reflective layer

SnO2 (or TiO2)

Chemically and mechanically resistant top layer Protective and anti-reflection coating Sub-oxidic layer High infrared reflectance layer Optimises the silver layer growing to be very thin Increases light transmittance of the low-e coating

Glass substrate

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Low-e window coatings can be designed with double or even triple metal layer films.39–42 These coatings also contain seed, anti-reflective and sacrificial layers but the silver film is separated into two or three thin layers that alternate with dielectrics. These types of multilayer dielectric/metal/ dielectric coatings serve as broadband IR reflective coatings. The coatings reduce thermal radiation losses through the glass and also provide control due to reduction of solar IR transmittance.40–42 Spectral characteristics of a standard clear and a low-iron glass and a low-e glazing are compared in Fig. 7.1 for the solar radiation spectral range. Spectral transmittance and reflectance of double and triple silver low-e glazings compared to a single silver low-e glass are shown in Figs 7.2 and 7.3.

Spectral transmittance (–)

(a) 1.0 0.8 0.6

Float glass Low iron glass Low-e glass

0.4 0.2 0.0 300

500

700

900 1100 1300 1500 1700 1900 2100 2300 2500 Wavelength (nm)

Spectral reflectance (–)

(b) 1

0.8

0.6 Float glass Low iron glass Low-e glass

0.4

0.2

0 300

500

700

900 1100 1300 1500 1700 1900 2100 2300 2500 Wavelength (nm)

7.1 Spectral transmittance (a) and reflectance (b) of a clear float and low iron and low-e glass. (Saint Gobain).

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Transmittance / reflectance (–)

1 0.9 0.8 0.7 0.6

Transmittance low-e (single Ag) Transmittance low-e (double Ag) Reflectance low-e (single Ag) Reflectance low-e (double Ag)

0.5 0.4 0.3 0.2 0.1 0 300

500

700

900 1100 1300 1500 1700 1900 2100 2300 2500 Wavelength (nm)

7.2 Spectral transmittance and reflectance of a low-e glass with single and double Ag layer. (Saint Gobain).

Transmittance / reflectance (–)

1 0.9 0.8 0.7 Transmittance low-e (single Ag) Transmittance low-e (triple Ag) Reflectance low-e (single Ag) Reflectance low-e (triple Ag)

0.6 0.5 0.4 0.3 0.2 0.1 0 300

500

700

900 1100 1300 1500 1700 1900 2100 2300 2500 Wavelength (nm)

7.3 Spectral transmittance and reflectance of a low-e glass with single and triple Ag layer. (Saint Gobain).

Spectrally selective multilayer films of polymers with different refractive indices represent another possibility for spectrally selective solar reflecting interference coatings. The coatings have colour neutrality and high light transmittance and IR reflectance in the spectral range between 780 and 2500 nm. They should be protected by a low-iron glass pane without any other coating on the exterior side.43

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All of the previously mentioned spectrally selective coatings can be classified as passive functional coatings with invariable optical properties.

7.3

Dynamic smart glazings

Glazings with dynamic variations of solar transmittance depending on external stimuli are known as ‘smart’ glazings. They reflect or absorb the infrared part of solar radiation to minimise solar heat gains. Smart glazings use chromogenic materials to facilitate reversible darkening and bleaching,44–49 and for this reason they are also called chromogenic glazings. The dynamic variations in glass transmittance can be initiated by changes of temperature in thermochromic glazings or solar radiation in photochromic glass. Electrically activated chromogenic glazings are represented by a group of switchable devices such as electrochromic glazings and their possible modifications, as well as liquid crystal glazings and suspended particle devices. The electrically activated glazings use transparent conductive oxide coatings to establish electric fields when connected to a circuit.

7.3.1 Electrochromic glazings Electrochromic glazings are chromogenic devices that dynamically modify transmittance under voltage. The electrochromic devices are coloured due to an electrochemical reaction in the electrochromic thin films.50–64 In the case of cathodic films, for example WO3 or Nb2O5, the cause is electrochemical reduction; whereas in anodic films as IrO2 or NiOOH or V2O5, the film becomes coloured due to electrochemical oxidation.48–55 The typical structure of an electrochromic glazing with a cathodic film is presented in Fig. 7.4.61,62 The electrochromic layer (WO3), ion conductor layer (LiAlF4) and ion storage layer (LixV2O5) are sandwiched between two glass panes with transparent conductive oxides. The conductive layers are connected with a low voltage source. The applied voltage causes an electrochemical reaction within the ion conductor layer, or electrolyte. Ions from the ion storage layer are driven through the electrolyte into the electrochromic layer in the darkening state. Li+ ions inserted into the electrochromic WO3 layer induce absorption. The insertion (colouration) and extraction (bleaching) of the lithium ions is in balance with an equal and opposite movement of electrons (e−) as the reaction progresses from the transparent state to the coloured state WO3 + ye− + yLi+ ↔ LiyWO3. Upon reversal of the applied potential, the ions migrate through the electrolyte into the ion storage layer and electrons are transported from the electrochromic layer via an external circuit.60–62 Some electrochromic glazing devices have the ion storage layer replaced by a complementary electrochromic layer.62–64 In this type of glazing, one

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Nanocoatings for architectural glass Colouration

Glass Transparent conductive oxide

In2O3: SnO2

Electrochromic layer Ion conductive layer Ion storage layer

WO3 Low voltage source

LixV2O5

Transparent conductive oxide

Bleaching

–

+ e

LiAlF4

189

–

Li+ Li+ e–

In2O3: SnO2 +

–

Glass

7.4 Example of electrochromic glazing composition (cathodic colouration).

Colouration

Glass Transparent conductive oxide

In2O3: SnO2

Electrochromic layer – anodic colouration Ion conductive layer Electrochromic layer – cathodic colouration Transparent conductive oxide

NiO LiAlF4 WO3

Oxidised state Low voltage source

Bleaching

Reduced state

Reduced state Oxidised state

In2O3: SnO2

Glass

7.5 Example of glazing composition with two complementary electrochromic layers.

electrochromic layer causes cathodic colouration (coloured in the reduced state) and the complementary electrochromic layer exhibits anodic colouration (coloured in the oxidised state). The first electrochromic layer is reduced and the second oxidised by the electrical activation. For example, a tungsten trioxide electrochromic layer is coloured in the reduced state and a nickel oxide layer is coloured in the oxidised state (see Fig. 7.5). The application of both electrochromic films supplies the desired level of optical modulation.63,64 When the tungsten oxide layer is in the oxidised state, the nickel oxide layer is reduced, resulting in high transparency.

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7.3.2 Photovoltaic-powered electrochromic glazings Photovoltaic-powered electrochromic glazings combine both the photovoltaic and electrochromic thin films in one switchable device (see Fig. 7.6). The photovoltaic layers generate a voltage which causes colouration of the electrochromic film. A transparent, conductive layer between the photovoltaic and electrochromic layers can be used to charge an external battery. The top and bottom transparent conductive layers are used for circuit connection.65–67

7.3.3 Gasochromic–electrochromic glazings Colouration of gasochromic glazings is chemically activated.61–64,68–70 A gasochromic device consists of an electrocatalytic thin coating based on tungsten trioxide deposited on glass. The glass is then sealed into a double glazed unit (the WO3 layer with the catalytic film facing the inside of the unit cavity). The device transmittance variations occur when either diluted hydrogen or diluted oxygen, which activates the switching, is introduced into the cavity. A chemical reaction of hydrogen with the catalytic film on the WO3 layer causes blue colouration. The reverse process (bleaching) is activated when the cavity is filled with diluted oxygen.61 Application of the gasochromic and electrochromic systems in one device represents an alternative to the electrically activated switchable glazing.62–64,68–70 The double glazed unit has both clear (transparent conductive) and electrochromic (electrochromic and ion conductive) layers, with the electrocatalytic coating exposed to hydrogen, as seen in Fig. 7.7. The gas reacts with the catalyst that serves as the source of ions. The electrocatalytic

Transparent conductive layer In2O3: SnO2 Electrochromic layer WO3 Ion conductive layer LiAlF4

Electrochromic part

Ion storage layer Li1.2V2O5 Transparent conductive layer In2O3: SnO2 (battery) PV layer p-type a-SiC:H PV layer intrinsic SiC:H

Photovoltaic part

PV layer n-type a-SiC:H Transparent conductive layer SnO2:F Glass substrate

7.6 Photovoltaic-powered electrochromic device.

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Low voltage source

Nanocoatings for architectural glass Gasochromic part

Glass Gas cavity with diluted hydrogen Electro-catalytic thin film

Electrochromic part

Ion conductor layer Electrochromic layer Transparent conductive layer

191

Low voltage source

Glass

7.7 Composition of gasochromic electrochromic device.

layer is connected with the transparent conductive layer and voltage is activated between them. Ions formed at the catalyst move through the ion conductive layer and are deposited into the electrochromic layer, which causes colouration. The bleaching process of the ion reversible transport is activated under reverse electric potential.

7.3.4 Photochromic-electrochromic glazing devices Photochromic glazings change their optical properties in response to radiation exposure, meaning that electrical activation is not required for colouration. Photosensitive materials are dispersed inside the glass or deposited as surface coatings.71,72 Dispersed silver halide crystals in photochromic glass create colour centres which are activated under ultraviolet and short-wave visible radiation. Long-wave visible radiation triggers destruction of the colour centres so that the photochromic glass becomes transparent. A photo-electrochromic glazing combines photochromic and electrochromic technology in one device.73–75 A layer of electrolyte such as a lithium solid polymer containing lithium iodide is placed between a titanium dioxide and an electrochromic layer. The porous TiO2 layer is dyeimpregnated.75 This three-layer thin film is sandwiched into two glass panes with transparent conducting oxide layers (see Fig. 7.8). The glazing colouration is solar-activated: the dye absorbs solar radiation and releases electrons. The electrons are injected into the porous TiO2 layer and move into the conducting oxide layer. They then pass through an external circuit to the conductive oxide layer adjacent to the electrochromic layer. The following flow of electrons through the central electrolyte towards the TiO2 layer causes lithium ions to be injected into the electrochromic layer of WO3 which causes colouration. Lithium ions are driven out of the electrochromic layer during the reversible bleaching process when solar rays do not affect the device.75 Despite the non-electrical activation system, external voltage can be applied to the circuit for user control of the device.

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Nanocoatings and ultra-thin films Glass Counter electrode - transparent conductive oxide Electrochromic layer WO3 Ions

Electrolyte Dye TiO2

Photo electrode – transparent conducting oxide

Electrons

Glass

7.8 Activated photoelectrochromic device.

Glass

Electrically activated state

Transparent conductive oxide Liquid crystals or Suspended particles

Low voltage source

Non-activated state

Diffusive semi-transparent (opal colour) liquid crystals layer or Transparent

Transparent conductive oxide

Dark coloured suspended particles

Glass

7.9 Typical composition of a glass laminate with liquid crystals or suspended particles.

7.3.5 Liquid crystal and suspended particle glazings Devices containing liquid crystals and suspended particles are similar to the glazing technologies already discussed, but the difference between them is light transmittance control. Liquid crystal glass is a light scattering device, while colouration of suspended particle glazing is due to light absorption (see Fig. 7.9).76–84 Liquid crystal glazing consists of a polymer matrix of microscopic cavities filled with liquid crystal molecules sandwiched between two glass panes, with transparent conductor layers connected to an external circuit.77–79 Liquid crystal molecules align in planes perpendicular to the transparent conductor layers when an alternating electrical field is activated between the conductors. In its electrically activated state, the glazing is transparent. In the off-state, the liquid crystals’ random configuration scatters light transmitted though the glazing.

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The suspended particle device consists of two glass panes with transparent conductive layers and an internal layer of organic fluid with suspended particles. The suspended particles are inorganic opaque randomly configurated molecules that absorb light. They can align in the presence of an electric field activated between the devices’ two transparent electrodes.76–78 Common liquid crystal and suspended particle glazing requires voltage in order to become transparent. A new self-switching liquid crystal glazing technology has been developed as a reverse mode, and works by doping the polymer liquid crystal matrix with photoconductive molecules.83,84

7.3.6 Thermally-activated glazings The light transmittance of thermotropic and thermochromic glazing depends on variations in temperature. The two types of glazing use different methods of solar control: thermotropic glazing is a light scattering device, whereas thermochromic glass is reflective in its activated state. Thermotropic glazing consists of two glass panes laminated with an internal gel layer. The gel is a mixture of two components with different refractive indices.85–87 In the transparent state the components are homogenous. When temperature rises the components are separated and transmitted light is diffused.87 Unlike liquid crystal technology, thermotropic glass does not require an external voltage source. Solar control in thermochromic glass is achieved due to a material phase transformation in the glass coating. The transformation changes the material properties between the semiconductor and metallic phases.88–94 Glass with a thermochromic coating is transparent in the semiconductor state but changes to become highly reflective in the metallic state. The reversible phase transformation is achieved using a doped vanadium dioxide VO2 coating. Tungsten92,93 or magnesium94 serves as the dopant. Thermochromic and multilayer coatings of alternating TiO2 and VO2 layers95 have been investigated for window coating applications.

7.3.7 Light control and thermal imaging glazings Combination of chromogenic glazing technology with optical systems provides smart transparent devices for solar control or conversion and light imaging applications. This can be achieved using holographic thin films and fluorescent materials96–98 as well as mirrors and prismatic surfaces that concentrate and redirect light.99,100 New technologies also include light-emitting diodes (LEDs) embedded in glass laminates. Organic LEDs are used for large flexible transparent sheets.101 Electrodes composed of carbon nanotubes and magnetic nanowires are used in glazed panels with electroluminescence effects.102

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Anti-reflective coatings are used for switchable glazing to enhance light transmittance in the transparent state as well as for many solar applications and architectural purposes. Coatings of a low refractive index material with high porosity or with surface nanopatterns, as well as multilayer dielectric films alternating materials with high and low refractive indices, can be used to reduce glass reflectance.103–111

7.4

Glass surface protections

Special glass coating technologies can also serve a protective purpose, improving abrasion and stain resistance of the glass and facilitating disinfection and cleaning.112–122 Ion beam milling is used to smooth the surface of an abrasion-resistant glass, and a diamond-like carbon coating is pyrolytically deposited on the surface. A similar technology can be used to create a stain-resistant surface. Surface sodium and sulphur concentrations are modified on the ion beam milled surface in an inert gas atmosphere. After this procedure a hydrophobic coating made up of a diamond-like carbon layer, and a fluoro-alkyl silane compound is deposited.101,114 Coatings containing dispersed silver ions serve an anti-bacterial function. The silver ion coating is convenient to use as a surface protection against bacteria or fungi.115 Another coating of SiO2 mixed with water or ethanol and known as ‘liquid glass’ can be applied to the surface to form an easy to clean and protective layer.116 Some coatings can reduce glass cleaning maintenance costs and prevent the glass from fogging. They can be classified as passive hydrophobic or hydrophilic coatings and active photocatalytic coatings that are activated due to solar radiation exposure.117–122 A glass hydrophobic coating contains silica nanoparticles embedded in an organic polymer matrix that creates a surface with nanoscale pores. The self-cleaning effect is evident.119,120 Spherical drops are created on the hydrophobic surface due to water surface tension, meaning that the contact area of water droplets with the surface is minimised. The principle of artificial hydrophobic surfaces is based on natural observation (lotus plant leaves, grass, etc). The higher contact angle the higher the hydrophobicity of a surface. Surfaces with a contact angle less than 90 ° are considered to be hydrophilic and the other surfaces are referred to as hydrophobic (see Fig. 7.10).119,120 The glass self-cleaning effect based on a combination of hydrophilic and photocatalytic function is provided by a TiO2 coating. The coating photocatalytically decomposes organic parts on its surface if activated under ultraviolet radiation. UV rays of solar radiation on the TiO2 layer excite titanium and oxygen electrons migrate into the organic material on the

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θ

θ

(a)

(b)

7.10 Water drop let contact angle on (a) hydrophilic surface θ < 90 °, (b) hydrophobic surface θ > 90 °.119

coating.121,122 Rain water reacts with oxygen vacancies on the glass surface. The surface become hydrophilic and washes away the dirt. A very special thin layer with carbon nanotubes can be applied as a protective layer to prevent condensation on glass. The layer connects to a source of electric current, creating a thin planar heating appliance that uniformly heats the glass area. The thin film has minimal heat capacity, meaning that heat is conducted into the glass substrate.123

7.5

Conclusion

Spectrally selective glazings for the reduction of heat radiation losses and solar infrared gains have wide architectural applications. Low emissivity glass coatings based on dielectrics with double or triple silver films provide light transmittance and more efficient reduction of the solar cooling load, compared to single silver low-e coatings. Chromogenic glazings are dynamic solar control devices that respond to electrical, chemical, thermal or radiation triggers. Electrochromic glazings were developed through several design modifications and have been combined with integrated photovoltaics or gasochromic and photochromic systems. Thermochromic glazing is a convenient material for solar control in architectural structures. Liquid crystals and suspended particle devices are also used for interior glazing purposes. Smart multifunctional windows, consisting of spectrally selective and switchable glazings with photovoltaics in combination with light-enhancing and solar concentration or redirection systems, can be used to create selfpowered, glazed devices for energy efficiency and visual comfort in buildings. Improving the durability of smart window devices, as well as reducing production and maintenance costs, is an important task for future developments in architectural glazing.

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7.6

Acknowledgements

The architectural glass coating review was completed within the frame of research projects MSˇMT MEB 080804 and GACˇR 101/09/H050. The author thanks the Saint-Gobain Glass Solutions CZ for the spectral characteristics of flat glass.

7.7

References

1. Lampert CM and Ma Y-P (1992), Advanced Glazing Technology: Fenestration 2000, Project-Phase III: Glazing Materials, LBL-31616, Berkeley, CA: LBNL. 2. Hutchins MG (1998), ‘Advanced glazing materials’, Sol. Energy, 62, 145–147. Robinson PD, Hutchins MG (1994), ‘Advanced glazing technology for the low energy buildings in the UK’, Renewable Energy, 5, 298–309. 3. Johnson ET (1991), Low-E Glazing Design Guide, Boston, MA: Butterworth Architecture. 4. Ander GD (2003), Daylighting Performance and Design, New York: Wiley, 12. 5. Born M and Wolf E (2003), Principles of Optics, Cambridge: Cambridge University Press. 6. Knittl Z (1976), Optics of Thin Films, New York: Wiley. 7. Bauchot M (2001), ‘Energy, environmental and economic benefits from advanced double glazing in EU dwellings’, Glass Performance Days, 18–21 June, Tampere, 822–825. 8. Smith GB et al. (1998), ‘Angular selective thin film glazing’, Renewable Energy, 15, 183–188. 9. Granqvist CG (2010), ‘Optical coatings for glazings’, Journal de Physiqué IV, 3, 1367–1376. 10. Lee ES, Selkowitz SE et al. (2006), Active Load Management with Advanced Window Wall Systems: Research and Industry Perspectives, Final Project Report CEC-500-2006-052-AT1, Berkeley, CA: LBNL. 11. O’Shaughnessy D (2009), ‘TH18-global megatrends and next-generation architectural glass’, Construct 2009, 16–19 June, Indianopolis. 12. Sakoske G et al. (2009), ‘High performance nano-coatings for glass’, Glass Performance Days, 15–16 June, Tampere, 169–172. 13. nano.DE-Report 2009, Federal Ministry of Education and Research, Bonn. 14. BSI (1998), BS EN 410:1998 Glass in Building – Determination of luminous and solar characteristics of glazing, London: British Standards Institution. 15. ISO (2010), ISO 11382:2010 Optics and photonics – Optical materials and components – Characterization of optical materials used in the infrared spectral range from 0,78 μm to 25 μm, Geneva: International Standards Organization. 16. Pulker HK (1998), Coatings on Glass, Amsterdam: Elsevier. 17. Incropera FP and Witt DP (1996), Fundamentals pf Heat and Mass Transfer, New York: Wiley. 18. Modest MF (2003), Radiative Heat Transfer, London: Academic Press.

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19. BSI (2010), BS EN 673 Glass in building. Determination of thermal transmittance (U value). Calculation method, London: British Standards Institution. 20. Terry Hollands KG et al. (2001), ‘Glazings and coatings’, in Gordon J (ed.), Solar Energy: The State of the Art, ISES Position Papers, London: James James, 53–69. 21. Berning PH (1983), ‘Principles of design of architectural coatings’, Appl Opt, 22, 4127–4141. 22. Karlsson B (1981), ‘Materials for solar transmitting heat reflecting coatings’, Thin Solid Films, 86, 91–98. 23. Bräuer G (1999), ‘Large area glass coating’, Surf Coat Technol, 112, 358– 365. 24. Lampert CM (1981), ‘Heat mirror coatings for energy conserving windows’, Sol Energy Mater, 6, 1–41. 25. Stjerna et al. (1994), ‘Optical and electrical properties of radio frequency sputtered tin oxide films doped with oxygen vacancies, F, Sb, or Mo’, J Appl Phys, 76, 3797–3817. 26. Hamberg I and Granqvist CG (1986), ‘Evaporated Sn-doped In2O3 film: Basic optical properties and applications to energy efficient windows’, J Appl Phys, 60, R123–R160. 27. Jin ZC et al. (1988), ‘Optical properties of sputter-deposited ZnO:Al film’, J Appl Phys, 64, 5117–5131. 28. Palik ED (1991), Handbook of Optical Constants of Solids, New York: Academic Press. 29. Valkonen E et al. (1984), ‘Solar optical properties of thin films of Cu, Ag, Au’, Sol Energy, 32, 211–222. 30. Fan et al. (1985), US Patent 4556277 Transparent Heat-Mirror. 31. Fan JC (1981), ‘Sputtered Films for wavelength-selective applications’, Thin Solid Films, 80, 125–136. 32. Szczyrbowski J et al. (1999), ‘New low emissivity coating based on TwinMag® sputtered TiO2 and Si3N4 layers’, Thin Solid Films, 351, 254–259. 33. Lampert CM (1981), ‘Heat mirror coatings for energy conserving windows’, Sol Energy Mater, 6, 1–41. 34. Schaefer C et al. (1997), ‘Low emissivity coatings on architectural glass’, Surf Coat Technik, 93, 37–45. 35. Martin-Palma RJ et al. (1998), ‘Silver-based low-emissivity coatings for architectural windows: optical and structural properties’, Sol Energy Mater Sol Cells, 53, 55–66. 36. Miyazaki M and Ando E (1994), ‘Durability improvement of Ag-based lowemissivity coatings’, J Non-Cryst Solids, 178, 245–249. 37. Kim D (2010), ‘Low temperature deposition of transparent conducting ITO/ Au/ITO films by reactive magnetron sputtering’, Appl Surf Sci, 256, 1774–1777. 38. Keel T, Holliday R and Harper T (2010), Gold and Nanotechnology in the Age of Innovation, London: World Gold Council. 39. Gläser HJ (2006), ‘History of the development and industrial production of low-e coatings for high heat insulating glass units’, Interpane 2006, available at: http://www.interpane.com/m/en/history_of_low-e_coatings_123.87.html (accessed April 2011).

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40. Glenn D et al. (2009), US Patent 7632572 B2 Double silver low emissivity and solar control coating. 41. Glenn D et al. (2003), US Patent Application Publication 2003/0049464 A1 Double silver low emissivity and solar control coating. 42. Hartig KW et al. (1996), US Patent 5557462 Dual Silver Low-e Glass Coating System and Insulating Glass Made Thereform. 43. Boettcher JA (2003), ‘Laminate performance results of metal free and color neutral solar reflecting film’, Glass Processing Days, 15–18 June, Tampere, 538–539. 44. Lampert CM (2004), ‘Chromogenic smart materials’, Mater Today, 7, 28– 35. 45. Granqvist CG et al. (2009), ‘Progress in chromogenics: New results for electrochromic and thermochromic materials and device’, Solar Energy Materials and Solar Cells, 9 (12), 2032–2039. 46. Lampert CM (1995), ‘Chromogenic switchable glazing: towards the development of the smart window’, Window Innovations ’95, 5–6 June, Toronto, 1–19. 47. Georg A et al. (1998), ‘Switchable glazing with a large dynamic range in total solar energy transmittance (TSET)’, Sol Energy, 62, 215–228. 48. Lampert CM and Granqvist CG (1990), Large-Area Chromogenics: Materials and Devices for Transmittance Control, Vol. IS4. Bellingham, WA: SPIE. 49. Schwarz M (2008), Smart Materials, Boca Raton, FL: CRC Press, Taylor & Francis. 50. Lampert CM (2002), ‘Electrochromics–history, technology, and the future – gasochromics’, in Chowdari, B. et al. (eds), Solid State Ionics: Trends in the New Millenium. Proceedings of the 8th Asian Conference, London: World Scientific, 411–422. 51. Granqvist CG (1995), Handbook of Inorganic Electrochromic Materials, Amsterdam: Elsevier. 52. Monk P et al. (2007), Electrochromism and Electrochromic Devices, Cambridge: Cambridge University Press. 53. Deb SK et al. (1978), US Patent 4120568 Electrochromic cell with protective overcoat layer. 54. Granqvist CG (2005), ‘Electrochromic device’, J Eur Ceram Soc, 25, 2907–2912. 55. Granqvist CG (1992), ‘Electrochromism and smart window design’, Solid State Ionics, 53–56, 479–489. 56. Granqvist CG (2008), ‘Oxide electrochromics: Why, how, and whither’, Sol Energy Mater Sol Cells, 92, 203–208. 57. Granqvist CG et al. (2007), ‘Nanomaterials for benign indoor environments: Electrochromics for “smart windows”, sensors for air quality, and photo-catalysts for air cleaning’, Sol Energy Mater Sol Cells, 91, 355–365. 58. Granqvist CG et al. (2003), ‘Electrochromic coating devices: survey of some recent advances’, Thin Solid Films, 442, 201–211. 59. Granqvist CG (2000), ‘Electrochromic tungsten oxide films: Review of progress 1993–1998’, Sol Energy Mater Sol Cells, 60, 201–262. 60. Granqvist CG et al. (1997), ‘Towards the smart window: progress in electrochromics’, J Non-Cryst Solids, 218, 273–279.

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61. Lampert CM (2004), ‘Chromogenic smart materials’, Mater Today, 7, 28– 35. 62. Cronin JP, Gudgel TJ, Kennedy SR, Agrawal A and Uhlmann, DR (1999), ‘Electrochromic glazing’, Mat Res, 2 (1), 1–9. 63. Nagai J and Seike T (1998), US Patent 5,721,633 Electrochromic Device and Multilayer Glazing. 64. Cerc Korosˇec R and Bukovec P (2006), ‘Sol–gel prepared NiO thin films for electrochromic applications’, Acta Chim Slov, 53, 136–147. 65. Benson DK and Branz HM (1995), ‘Design goals and challenges for a photovoltaic-powered electrochromic window covering’, Sol Energy Mater Sol Cells, 39, 204–211. 66. Gao W et al. (2000), ‘Approaches for large-area a-SiC:H photovoltaicpowered electrochromic window coatings’, J Non-Cryst Solids, 266–269, 1140– 1144. 67. Gao W et al. (1999), ‘First a-SiC:H photovoltaic-powered monolithic tandem electrochromic smart window device’, Sol Energy Mater Sol Cells, 59, 243–254. 68. Wilson HR et al. (2002), ‘The optical properties of gasochromic glazing’, in Klages C-P and Glaser HJ (eds), International Conferences on Coatings on Glass and Plastics, 3–7 November, Braunschweig, 469–657. 69. Wittwer V et al. (2004), ‘Gasochromic windows’, Sol Energy Mater Sol Cells, 84, 305–314. 70. Wittwer V and Graf W (2001), ‘Gaschromic glazings with a large dynamic range in total solar energy transmittance’, Glass Performance Days, 18–21 June, Tampere, 725–728. 71. Araujo RJ (1980), ‘Photochromism in glasses containing silver halides’, Contemp Phys, 21, 77. 72. Chu N (1990), ‘Photochromic plastics’, in Lampert CM and Granqvist CG (eds), Large-Area Chromogenics: Materials and Devices for Transmittance Control, Vol. IS4, Bellingham, WA: SPIE, SPIE Bellingham, 102–121. 73. Teowee G et al. (2001), US Patent 6246505 Photochromic Devices. 74. Yoshimura K and Okada M (2007), US Patent 7259902 Reflective Light Control Element with Diffusible Reflecting Surface. 75. Gregg BA (1997), ‘Photoelectrochromic cells and their applications’, Endeavour, 21(2), 52–55. 76. Amstock JS (1997), Liquid Crystals and Suspended-Particle Device. Handbook of Glass in Construction, New York: McGraw-Hill. 77. Macrelli G (1995), ‘Optical characterization of commercial large area liquid crystal devices’, Energy Mater Sol Cells, 39 (2–4), 123–131. 78. Lampert CM (1993), ‘Optical switching technology for glazing’, Thin Solid Films, 236, 6–13. 79. Wigginton M (1996), Glass in Architecture, London: Phaidon Press. 80. Sucheol P and Hong JW (2009), ‘Polymer dispersed liquid crystal film for variable-transparency glazing’, Thin Solid Films, 517, 3183–3186. 81. Sixou P et al. (2001), ‘Switchable liquid-crystal/polymer micro-composite glazings’, Glass Processing Days, 18–21 June, Tampere, 729–734. 82. Garnier DJ et al. (2009), ‘High-efficiency multistable switchable glazing using smetic A liquid crystals’, Sol Energy Mater Sol Cells, 93, 301–306.

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83. Cupelli D et al. (2009), ‘Self-adjusting smart windows based on polymer-dispersed liquid crystals’, Sol Energy Mater Sol Cells, 93, 2008–2012. 84. Cupelli D et al. (2004), ‘Fine adjustment of conductivity in polymer-dispersed liquid crystals’, Appl Phys Lett, 85, 3292–3294. 85. Wilson HR (1994), ‘Optical properties of thermotropic layers’, Proc. SPIE, 2255, 473–484. 86. Seeboth A et al. (2004), ‘Chromogenic polymer gels for reversible transparency and color kontrol’, in Samson A et al. (eds), Chromogenic Phenomena in Polymers, ACS Symposium Series, Vol. 888, 110–121. 87. Nitz P and Hartwig H (2005), ‘Solar control with thermotropic layers’, Sol Energy, 79, 573–582. 88. Day J and Willet R (1990), ‘Science and technology of thermochromic materials’, in Lampert CM and Granqvist CG (eds), Large-Area Chromogenics: Materials and Devices for Transmittance Control, Vol. IS4, Bellingham, WA: SPIE, 122–147. 89. Jorgenson GV and Lee JC (1990), ‘Thermochromic materials and devices: inorganic systems’, in Lampert CM and Granqvist CG (eds), Large-Area Chromogenics: Materials and Devices for Transmittance Control, Vol. IS4, Bellingham, WA: SPIE, 142–159. 90. Babulanam SM et al. (1987), ‘Thermochromic VO2 films for energy efficient windows’, Sol Energy Mat, 16, 347–363. 91. Jorgenson GV and Lee JC (1986), ‘Doped vanadium oxide for optical switching films’, Sol Energy Mat, 14, 205–214. 92. Parkin I and Manning T (2007), US Patent 0048438 Thermochromic coatings. 93. Blackman Ch et al. (2009), ‘Atmospheric pressure chemical vapour deposition of thermochromic tungsten doped vanadium dioxide thin films for use’, Thin Solid Films, 517, 4565–4570. 94. Mlyuka NR et al. (2009), ‘Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature’, Appl Phys Lett, 95, 171909. 95. Granqvist CG et al. (2010), ‘Advances in chromogenic materials and devices’, Thin Solid Films, 518, 3046–3053. 96. Riccobono J and Ludman J (2002), ‘Solar holography’, in Ludman J et al. (eds), Holography for the New Millenium, New York: Springer, 157– 172. 97. Hans DT et al. (1993), ‘Design optimization and manufacturing of holographic windows for daylighting applications in buildings’, in Lampert CM (ed.), Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XII, Vol. 2017, Bellingham, WA: SPIE, 35–45. 98. Stojanoff CG (2006), ‘Engineering applications of HOEs manufactured with enhanced performance DCG films’, in Bjelkhagen HI and Lessard RA (eds), Practical Holography XX: Materials and Applications, Bellingham, WA: SPIE, 613601. 99. Hoßfeld W et al. (2003), ‘Application of microstructured surfaces in architectural glazings’, ISES Solar World Congress, 14–19 June, Göteborg. 100. Gunther W et al. (2005), ‘Combination of microstructures and optically functional coatings for solar control glazing’, Sol Energy Mater Sol Cells, 89, 233–248.

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101. Chamberlain H (2008), ‘Nanotechnologies expand glazing options and the complexity of specifications’, Building Enclosure Science and Technology, 10–12 June, Mineapolis. 102. Oak Ridge National Laboratory (2007), ORNL helps develop next-generation LEDs, press release, available at: http://www.ornl.gov/info/press_releases/get_ press_release.cfm?ReleaseNumber=mr20070319-00 (accessed May 2011). 103. Boire P et al. (2000), US Patent 6086914 Glazing Pane Having an AntiReflection Coating. 104. Hofmann T and Kursawe M (2003), ‘Antireflective coating on glass for solar applications glass’, Glass Performance Days, 15–16 June, Tampere, 382–384. 105. Olsson G (2003), ‘Low cost industrial manufacturing of a thin single layer antireflective surface on sheet glass’, Glass Performance Days, 15–16 June, Tampere, 340–341. 106. Southwell WH (1991), ‘Pyramid-array surface relief structures producing anti-reflection index matching on optical surfaces’, J Opt Soc Am A, 8, 549– 553. 107. Hammarberg E and Roos A (2003), ‘Antireflection treatment of low-emitting glazings for energy efficient windows with high visible transmittance’, Thin Solid Films, 442, 222–226. 108. Jonsson A and Roos A (2010), ‘Visual and energy performance of switchable windows with antireflection coatings’, Solar Energy, 84(8), 1370–1375. 109. Roos A et al. (2009), ‘Applications of coated glass in high performance energy efficient windows’, Glass Performance Days, 12–15 June, Tampere, 107–110. 110. Boire P et al. (2000), US Patent 6086914 Glazing Pane Having an Anti-Reflection Coating. 111. Gombert A et al. (1998), ‘Glazing with very high solar transmittance’, Solar Energy, 62, 177–178. 112. Provder T and Baghdachi J (2007), Smart Coatings. ACS Symposium Series 957, Washington DC: American Chemical Society, Oxford University Press. 113. Provder T and Baghdachi J (2009), Smart Coatings II. ACS Symposium Series 1002, Washington DC: American Chemical Society, Oxford University Press. 114. Anderson JS (2001), US 2001/0044027 A1 Diamond-like Carbon Coating on Glass for Added Hardness and Abrasion Resistance. 115. Hecq A et al. Substrate with Antimicrobial Properties, Patent application number: 20090324990. 116. Technology Marketing Management (2009), “Liquid Glass” Nanotechnology Gives Big Savings in Cost and Environmental Impact for Cleaning Industry – Now Available in Cyprus, available at: http://www.technologymarketingmanagement.com/Index.asp?PageID=241 (accessed April 2011). 117. Armand P (2003), ‘Self-cleaning coatings for architectural application’, Glass Performance Days, 15–16 June, Tampere, 326. 118. Nakamura M et al. (2006), ‘Hydrophilic property of SiO2/TiO2 double layer films’, Thin Solid Films, 502 (1–2), 121–124. 119. Hohenstein H (2003), ‘Coatings with nano-particles for windows and façades’, Glass Performance Days, 15–16 June, Tampere, 338–339. 120. Zhang YL and Sundararajan S (2008), ‘Superhydrophobic engineering surfaces with tunable air-trapping ability’, J Micromech Microeng, 18, 1–7.

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121. Hüber M (2003), ‘TiO2-coatings from inorganic soles: a new approach to hydrophilic and photocatalytically active glasses’, Glass Performance Days, 15–16 June, Tampere, 328–329. 122. Gläser HJ (2005), ‘The effects of weather onto glazing and their influence’, Part II, Glass Processing Days, 17–20 June, Tampere, 1–9. 123. Fraunhofer (2006), Transparente Schicht für freie Sicht, Mediendienst 12, available at: http://www.fraunhofer.de/archiv/pi-2006/presse/presseinformationen/2006/12/Mediendienst122006Thema1.html (accessed May 2011).

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8 Nanocoatings and ultra-thin films for packaging applications A. SORRENTINO, University of Salerno, Italy

Abstract: The concept of packaging is under continuous review and development as it is key in gaining competitive advantage in modern industry. The original function of packaging as a means of protection and preservation against external contamination is in fact only one of several current market requirements. An innovative pack design can open up new distribution channels, providing improved presentation, incurring lower costs, ensuring safety and reliability and enhancing brand recognition. This chapter first offers a brief introduction to the principal packaging applications, and then presents a number of applications of nanomaterials in this sector. Finally, future trends are discussed, with some consideration given to safety concerns. Key words: nanomaterials in packaging, high barrier packaging, nanosensors in packaging, anti-static packaging applications, smart packaging.

8.1

Introduction

Packaging technology is of strategic importance as it can be key to gaining competitive advantage in the modern industry.1–3 An innovative pack design can open up new distribution channels, providing improved presentation, incurring lower costs, increasing margins, enhancing brand recognition, enhancing product safety and integrity, and facilitating transportation and distribution.4 The packaging industry therefore faces the constant challenge of providing cost-effective pack performance, with health and safety being of paramount importance. At the same time, there is ongoing legislation and political pressure to reduce both the amount of packaging used and the amount of packaging waste.5 Every new type of packaging is thus the product of a wide range of influences, including quality, production, engineering, marketing, purchasing, legal issues, finance, logistics and environmental management. Figure 8.1 summarizes the principal objectives of packaging.1 Packaging is a means of ensuring safe delivery to the end consumer, preventing any mechanical damage due to hazards encountered during distribution. It is a system for preparing goods for transport, distribution, storage, retailing and end-use. Packaging is also the primary means of communicating all the necessary information to the various stages of the distribution chain, up to 203 © Woodhead Publishing Limited, 2011

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Security

Marketing

Int

ent ing ell

Act ive

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Communication

Convenience

Practical Containment 8.1 Principal packaging objectives.

and including the consumer (legal requirements, product ingredients, use, etc.). It is also the first opportunity for brand promotion (symbols, illustrations, advertising and color, free extra product, new product, money off). It is sometimes convenient to categorize packages as primary, secondary or tertiary, depending on function or use. Primary packaging is the material that is first wrapped around the product, containing it. This is usually the smallest unit of distribution or use and is the package which is in direct contact with the contents. Secondary packaging is outside the primary packaging, perhaps used to group primary packages together. Tertiary packaging is used for bulk handling, warehouse storage and transport shipping.1,2 It is evident that the performance levels of different types of packaging are extremely diverse, ranging from relatively no strength to extremely high strength, flexible to rigid, non-permeable to permeable, transparent to opaque, etc. Polymers are probably unique in being able to meet all of these requirements. These materials are also the most efficient due to their product-to-package ratio. Indeed, polymer materials are 50% more efficient than aluminum, 400% more efficient than paper and 1500% more efficient than glass.6 It is no coincidence that the packaging industry, together with the technology it employs, is the major consumer of plastics, accounting for about 30 wt % of all annual plastic production. Polymers are used in packaging in a variety of forms: single films, multilayer flexible structures, sheets, coatings, adhesives, foams, laminations and rigid or semi-rigid containers. The most widely used plastics in flexible packaging are low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and polypropylene (PP). High density polyethylene (HDPE) is widely used for rigid containers. However, the fastest-growing plastic for rigid containers is polyethylene terephthalate (PET) and its copolymers. Other resins such as ethylene vinyl alcohol (EVOH), polyvinyl chloride (PVC) copolymers and nylons have specialized applications as high barrier materials.6 Sustainable packaging involves technologies that are environmentallyfriendly, socially acceptable, and economically viable; for example, a sustain-

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able package may be made of bio-based materials that are biodegradable and inexpensive but that still possess the properties required for the specific application. Different designs and processing techniques are used to produce packaging products. Of these, the most important are extrusion, used for the films, and thermoforming, used for sheets. Injection molding is used for rigid containers while blow molding is used for producing bottles and cups. In the following section, some of the more important and representative packaging applications are presented and discussed.

8.1.1 Food packaging Food packaging can be defined as the means of achieving safe delivery of products in sound condition to the final user at a minimum cost.3 It provides a barrier between the food and the environment by controlling the transmission of light, the rate of transfer of heat, moisture and gases, and the movement of microorganisms or insects.7 The packaging materials should be easy to handle, efficient and economic on the production line, resistant to damage, safe and not harmful to human health.8 Packaging lowers the cost of foods through economies of scale in mass production and efficiency in bulk distribution (Fig. 8.2). Savings are also derived from reduced product damage.

8.2 Examples of food packaging.

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The main difference between food packaging and other type of packaging is related to the constant activity of the food; in fact, it is frequently observed that foods ‘breathe’ during storage. Thus, the success of a packaging material for food products is entirely dependent on its ability to control the internal gas composition (i.e. oxygen, ethylene, carbon dioxide, etc.) and water loss.7,8 In some specific applications, however, the atmosphere around the product can be specifically changed to prolong the shelf life of the food.6,7 A packaging is defined as active when it interacts with the packed food and the environment and plays a dynamic role in food preservation. Developments in active packaging have led to advances in many areas, including delayed oxidation and controlled respiration rate, microbial growth and moisture migration. Other active packaging technologies include carbon dioxide absorbers/emitters, odor absorbers, ethylene removers and aroma emitters. Of these technologies, oxygen scavengers, moisture absorbers and barrier packaging today constitute more than 80% of the market.6

8.1.2 Pharmaceutical packaging applications Drugs require more care in their packaging than most other products. Any failure in their packaging could result in changes in the drug that could then lead either to delayed curative effects, or to illness or injury of the patient.9,10 Indeed, the loss of potency is the key factor in determining the shelf life of a pharmaceutical product. In some cases, the degradation of pharmaceutical products can be toxic. Here the rate of accumulation of toxic elements is of greater importance to the shelf life of the product than the loss of the active ingredient. In considering the stability of pharmaceutical products, it is essential to take into account the whole product, excipients, pack and label. For example, migration of a plasticizer from the plastic bottle into a label could cause the ink to become blurred so that the legibility of the information on the label is impaired.11 Compliance packaging such as that used for fixed-dose combination pills and unit-of-use packaging is a therapy-related intervention that is designed to simplify medication regimens.12 This type of packaging is usually based on blister packaging using unit-of-use dose (Fig. 8.3). The separate dosage units and separate days are usually indicated on the dosage cards to help remind the patient when to take the medication, and how much to take.13

8.1.3 Electronic packaging Electronic packaging refers to enclosures for integrated circuits, passive devices and circuit cards (Fig. 8.4). The effectiveness of an electronic system,

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8.3 Pharmaceutical packaging.

8.4 Electronic packaging products.

as well as its reliability and cost, is strongly determined by the packaging materials used.14 This is of fundamental importance for signal and power transmission, heat dissipation, electromagnetic interference shielding and protection from environmental factors such as moisture, contamination, chemicals and radiation.15,16

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Continued miniaturization, increased performance and increased reliability of microsystems all require the development of new materials and manufacturing methods. One of the major limitations to continued increases in performance in the semiconductor and power electronics industries is thermal management.1 There is a need for the industry to develop highly thermally conductive materials which facilitate effective heat dissipation, and to rapidly incorporate these materials into manufacturing processes. It is essential that microsystems are allowed to operate within their designated temperature interval. Operation at excessively high temperatures may cause a deterioration in semiconductor performance as well as fracture, delamination, melting, creep and corrosion.17,18

8.2

Nanomaterials in packaging

Nanomaterials are an emerging group of materials that have superior physical or mechanical properties compared with their conventional or bulk counterparts.19–21 Their properties were obtained by controlling the arrangements of matter at the atomic or molecular scale.22,23 Due to the dramatically increased surface area, nanometer-size grains, fibers and plates show chemical and physical properties completely different from those inherent to other conventional materials.24 In fact, a homogeneous dispersion of nanometer-size materials along with nanometric layer deposition can introduce significantly different properties not exhibited by the pristine materials.25 Nanotechnology has the capability to transform the nature of conventional packaging materials, to providing these materials with the desired properties.26,27 Traditional packaging will be replaced with multifunctional intelligent packaging thanks to the application of nanotechnology. Through the use of different nanostructures, materials can be developed that offer various levels of gas/water vapor permeability according to requirements. With the use of nanocoatings or nanoparticles, packaging can be made lighter and stronger with better thermal performance and less gas absorption.28 Furthermore, nanostructured packages can effectively prevent the invasion of bacteria and microorganisms and can ensure product safety. Embedded nanosensors in the packaging will allow consumers to check the exact status of the contents. Dirt repellent coatings for packages are also being developed. Simple packaging materials will become ‘smart,’ meaning that they will be able to respond to environmental conditions or to repair themselves. This will make products cheaper, and will lead to more efficient and sustainable production, thanks to the use of less water and fewer chemicals and the production of less waste.

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A nanostructured composite can be defined as a material that consists of two or more different material components, at least one of which has a dimension in the nanometer scale.29–31 There are many types of nanostructured composite currently being researched and developed, including polymer/inorganic particles, polymer/polymer coatings, metal/ceramic and inorganic-based composites.30,32–35 Nanostructured composites may be prepared from solvents in which both polymer and complex are dissolved.36 Another method involves the melt mixing process. In this case, the application of shear during compounding assists exfoliation and dispersion.37 In an alternative approach, also known as ‘in situ polymerization’, the nanolayer is deposited in the form of liquid monomers that must be polymerized directly on or inside the composite sheets. Polymerization can be initiated by heat or radiation, by the diffusion of a suitable initiator, or by an organic initiator or catalyst fixed through cationic exchange.38,39 The solid-state mixing at room temperature is a method for the preparation of nanocomposites that does not require the use of high temperature or solvent treatments. In this case, the dispersion of nanometric particles is promoted by the energy transfer between the milling tools and the composite particles mixture.40 Probably one of the most important advantages of these systems is that a relatively small amount of material is required, typically 0.1–5 wt %, as a result of the nanometric scale dimension.41 Therefore, these systems avoid many of the costly and cumbersome fabrication techniques common to conventional composites. Furthermore, they can be adapted for use in films, fibers and monoliths.42 The melt-compounding approach would be particularly well-suited to the production of nanostructured composites, since existing technologies and equipment could be utilized and scaled to commercial quantities.43,44

8.3

High barrier packaging

High barrier packaging can be broadly defined as a system that has the ability to protect goods from gases, vapors and liquids. Since all packaging materials restrict the transport of penetrants to some degree, it is difficult to provide a concise, objective definition. However, the barrier levels offered by the different materials are subject to considerable variation. For this reason, the selection of a barrier material for a particular application typically involves tradeoffs between permeation, mechanical and aesthetic properties as well as economic and recycling considerations.1,4 Several different approaches are used to provide permeability resistance in packaging.45 The most widely used of these is the combination of two or more polymers, or other materials.2 Through this method, it is possible to achieve performance advantages not offered by any of the materials when

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used in isolation. For plastic packaging, this approach generally involves a thin layer of an expensive high barrier polymer sandwiched between layers of less expensive, structural polymers. Unfortunately, barrier polymers display different permeation behavior with different gases and are extremely sensitive to the environmental conditions (Fig. 8.5). For this reason, as the barrier properties of a multilayer film are maximized for a particular gas (such as 02, N2, or C02), other properties such as mechanical and moisture resistance diminish.3,6,45–47 Polyethylene is considered to be the polymer of choice when a barrier against water vapor is required. It is frequently used to line the inside of food packaging films, ensuring that moisture cannot escape.48 Only two plastics were previously available for the prevention of these undesirable reactions: polyamide 6, which is inexpensive but somewhat more permeable, was used for less sensitive materials, and EVOH, more expensive but also more airtight, was used for sensitive products.1,49,50 The relatively inexpensive polypropylene is an excellent water vapor barrier, but a poor gas barrier, while polyethylene is a poor flavor barrier. A prominent biomass-derived plastic, polylactic acid (PLA), has relatively poor barrier properties, with low glass transition temperature and less strength than many hydrocarbon-origin plastics (Fig. 8.5). High barrier nanocoatings consist of hybrid organic–inorganic nanocomposite coatings of hybrid precursors and sol–gel systems (e.g. propyltrimethoxysilane (PTMO) and Bayresit®), and are being developed for active packaging applications such as foil treatment. The coatings are produced through atmospheric plasma technology using dielectric barrier discharges. The coatings have been reported to be very efficient at keeping out oxygen and retaining carbon dioxide, and can rival traditional active packaging

Water vapor permeability

1000 100

Nylon-6

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PAN HDPE

1 0.1

PET LCP

PP

PVDC

0.01 0.001 0.001

0.01

0.1

1

10

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Oxygen permeability

8.5 Oxygen permeability as a function of water vapor barrier properties for various polymers at 23 °C.

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technologies such as oxygen scavengers. Examples include InMat Nanocoatings, which are aqueous-based nanocomposite barrier coatings that provide an oxygen barrier with a few-micron coating. Another example is silica–polymer hybrids manufactured by the sol–gel process, which can also improve oxygen-diffusion barriers for plastics (such as PET) in the packaging industry. Chemical or physical modification consists of the modification of the surface of the plastic during or after manufacturing. One example of this is the plasma deposition treatment carried out on the surface of flexible film packaging before it is coated with high barrier paint. Another wellestablished technology employs chemical vapor deposition to coat a nanometer-thick layer of silicon oxide to produce enhanced barrier properties. In some cases, HDPE gasoline containers/tanks may also be chemically treated through exposure of their surface to sulfonation or fluorination. This means that the level of gasoline permeation through the polymer meets the strict standard requirements. Inorganic–organic composites materials, in which inorganic fillers are dispersed at the nanometric level in a polymeric matrix, have recently attracted attention as an alternative means of improving the barrier properties of polymers.51–54 Their ultrafine phase distribution has also been shown to improve other properties, such as flame resistance, high modulus, increased heat resistance, biodegradability, transparency and solvent resistance.55,56 The inorganic phase commonly used is made up of layered silicate clays that belong to the same general family of 2 : 1 layered or phyllosilicates.57–59 This clay has a natural platy structure with individual platelets 1 nm thick and surface lengths in the order of 100–1000 nm. Montmorillonite, one of the clay minerals more frequently used as polymer filler, is available as micron-size tactoids, consisting of several hundred plate-like structures held together by electrostatic forces.29,60 Layered double hydroxides (LDHs) represent another interesting class of potential nanofillers for polymers.61 LDH particles consist of magnesium aluminium hydroxide layers, which, in contrast to layered silicates, display a positive surface charge which is counterbalanced by anions located in the domains between adjacent layers.62 Both natural and synthetic clays are hydrophilic, which makes proper exfoliation and dispersion into conventional polymers difficult. The clays must therefore be modified through substitution of their sodium ions with organic ammonium ions, resulting in an organo-clay complex. The modification procedure expands the spacing between individual clay layers and improves the compatibility of the complex with the polymer so that individual platelets can be more easily separated in a polymer matrix. The subsequent incorporation of this hybrid into the polymer matrix causes the

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separation and dispersion of the individual clay particles in the matrix polymer.23,24 The degree of exfoliation of the clay in single layers is heavily dependent upon the processing conditions and on the characteristics of the polymer matrix.29 A number of studies have shown that polymers filled with impermeable nanoparticles have a lower permeability than that of the corresponding virgin polymer. The permeability of a polymer nanocomposite is defined as (Eq. 8.1): P = Dc Sc

[8.1]

where Dc and Sc are the diffusivity and the solubility values, respectively, averaged over the range of penetrant concentration inside the nanocomposites sample. The diffusion and solubility of the nanocomposites can exert very variable influences on the permeability. For example, it has frequently been observed that a decrease in the diffusion parameter of the nanocomposites is the most relevant factor in the decrease in permeability, even when the solubility is similar or higher. This phenomenon has traditionally been explained by considering the clay sheets as impermeable obstacles in the path of the diffusion process. The clay sheets act like barriers, which makes it difficult for gases or vapor to pass through the bulk material. The use of impermeable clay thus increases the distance that the gas molecules have to travel, by causing those molecules to zigzag around the silicate plates effectively increasing the amount of time it will take for the molecules to completely penetrate the barrier (Fig. 8.6). The aspect ratio of typical square platelet filler is defined as one lateral dimension divided by the thickness dimension. Obviously, in the fully exfoliated state, the number

Diffusion

L t

8.6 Models of aligned mono-disperse flakes in a periodic arrangement.

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of obstacles and the aspect ratio are at the highest possible level, and thus the greatest improvement in barrier performance is expected. In addition to the formation of sinuous pathways, the development of interface regions between polymer bulk and clay sheets can influence the barrier properties of the composite. The presence of these regions around the single slabs can occasionally be responsible for a diffusional enhancement, as observed in some experiments.22 Interface regions can be caused either by the surface modifiers utilized to make the inorganic sheets compatible with the polymer or by the formation of micro-voids between the different phases.63 The presence of these regions is responsible, for example, for the differences in the behavior of nanocomposites treated with different surface modifiers, and/or for the variable behavior shown by the same nanocomposites with respect to different permeant molecules. Conceptually, the higher diffusion coefficient in these zones will compensate for the increase in tortuosity of the diffusional path to some degree. The relative diffusivity can be expressed by the following relationship: Dc α = D0 τ

[8.2]

where Dc, the diffusivity in the composite polymer material, is reduced from its value in the neat polymer matrix, D0, by a geometric hindered factor τ and a vacancy factor α. The geometric hindered factor τ was introduced to account for the increased effective path length caused by the presence of the impermeable sheets. Its value can be calculated by geometric considerations using the idealized two-phase system, as shown in Fig. 8.6. The vacancy factor α was introduced to account for the variation in diffusion resulting from the possible formation of the interface zone between the inorganic sheets and the polymer matrix. The simpler, probably more famous model adopted to describe the enhancement in barrier properties due to the dispersion of nanometric filler is the Nielson equation.64 These models assume that the vacancy and the hindered factor in Eq. 8.2, are equal to: θ τ = ⎛⎜ 1 + χ⎞⎟ and α = 1 ⎝ 2 ⎠

[8.3]

In Eq. 8.3, θ is the aspect ratio of the particles (θ = width/thickness) and χ is the filler volume fraction. Experimentally, the validity of this model was supported by large reductions in effective permeability as a function of volume fraction and aspect ratios. A reduction in oxygen and carbon dioxide permeation of up to 80–90% has been reported with a relatively small mass of nanoclay, less than 5%, with an aspect ratio of 20 : 1.41

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A nominal aspect ratio can be calculated with this model by fitting the relative permeability data as a function of clay content. Even if the result is only approximate, this nominal aspect ratio has proven to be a good indicator for the extent of clay exfoliation. The quality of the interface region and its extension are clearly dependent on the type of organo-clay and its dispersion. In some applications, controlled microporous voids that allow permeation of gas (oxygen and carbon dioxide) are of great importance. In these cases, the selection of the correct type of organo-clay to be blended into the hydrophobic polymer matrix can allow the production of uniaxially oriented film. The microstructure of the film displays a sufficient number of elongated, narrow shaped voids that can, for example, delay respiratory anaerobiosis in fresh fruit and vegetables packaged in modified atmospheres. Durethan®, from Bayer Polymers, is a Nylon-6 nanocomposite which restricts the entry of gases and the loss of moisture, as well as retaining excellent transparency. This new film material created using nanoparticles is inexpensive and, although not as effective as EVOH, it offers better properties than simple polyamide 6. The minute particles influence the crystallization of the plastic, acting as nuclei for the crystallization of the polymer, thus improving the light diffusion through the film. This is a particularly useful example of a plastic with a lower price or lower performance that may be improved by the incorporation of nanoclays, and may then subsequently be suitable for use in applications from which they have been effectively precluded previously.

8.3.1 Oxygen scavenging materials Elevated O2 levels in food packages may facilitate both microbial growth and direct oxidation reactions of the product, leading to a significant reduction in its shelf life.65 Traditional vacuum packaging may not facilitate the complete removal of O2. Residual O2 may be removed using oxygen scavenging technology (Fig. 8.7). The existing technologies are based on several approaches including iron powder oxidation, ascorbic acid oxidation, photosensitive dye oxidation, enzymatic oxidation (e.g. glucose oxidase), unsaturated fatty acids, rice extract or immobilized yeast on a solid substrate.66 The oxygen scavenging materials are generally supplied in sachets and are introduced into the packaging together with the goods. The principal disadvantages of these technologies are first, that the reaction is moisture-dependent and second, that waste disposal after consumption is difficult. Nanocoatings can be applied directly to the packaging materials in order to maintain very low oxygen levels. One of the most promising candidates for future use in the production of polymeric films for packaging are titania

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8.7 Oxygen scavenging materials.

nanoparticles.67 The nanocrystalline titania is activated by a photocatalytic mechanism that requires the exposure of the packaging to the UVA light before use.67 Atomic layer deposition can be used to produce high quality thin layer coatings that have excellent barrier properties against water vapor and oxygen. Recent developments such as the continuous operation mode have made it possible to extend the use of this technique to new applications such as oxygen scavenger polymer films.

8.4

Anti-microbial packaging

Nanotechnology is also being used to develop active anti-microbial packages that could help to control the growth of pathogenic and spoilage microorganisms.68–72 Anti-microbial properties can be imparted to packaging through the incorporation of silver, magnesium oxide, copper oxide or zinc oxide nanoparticles which kill harmful microorganisms.73–77 For example, it is possible to prepare a multilayer packaging consisting of an outer layer of traditional film and a trap layer containing a nanoparticles system with a microbial inhibitor. Such a system is particularly desirable due to its acceptable structural integrity and barrier properties imparted by the multilayer structure, and to its anti-microbial properties contributed by the anti-microbial agents.78 It could also be designed to stop microbial growth once the package is opened by the consumer and rewrapped with an active-film portion of the package. It has long been known that silver prevents the growth of pathogens and spoilage bacteria. Surfaces in some refrigerators and dishwashers are

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treated with microscopic silver particles as they are highly toxic to a wide range of microorganisms, are not volatile and have good temperature stability.79,80 The silver particles proposed for use in packaging are in the nanometric scale.81–84 The higher surface-to-volume ratio allows nanomaterials to attach more copies of biological molecules, providing greater efficiency.85–90 In the presence of UV light, titanium dioxide generates reactive species that cause degradation of organic compounds and, potentially, bacteria.91–95 The photocatalysis of TiO2 has been used to inactivate several food-related pathogenic bacteria.96–99 The activity of TiO2 can be enhanced by the deposition of silver onto nanoparticles.95,100,101 Due to its intense anti-microbic action, silver-doped TiO2 nanoparticles are used in air filters and on aluminum surfaces. Metal doping improves the visible light absorbance of TiO291,96 and increases its photocatalytic activity under UV irradiation.101 It has been demonstrated that doping greatly improves photocatalytic bacterial inactivation. TiO2 doped with silver was used to obtain an anti-bacterial nanocomposite containing PVC.95 It has recently been discovered that carbon nanotubes might exhibit powerful anti-microbial effects: E coli bacteria died on immediate direct contact with aggregates of carbon nanotubes.102 TiO2 combined with carbon nanotubes provides enhanced disinfectant properties against Bacillus cereus spores.103 Treatment of the surface in contact with the goods along the whole production process can also help to reduce the possibility of contamination with microorganisms. Nanocoatings can be applied to machinery, pipes, heat exchangers and so on in order to prevent the buildup of deposits on their surfaces. In this way it is possible to reduce both the levels of bacteria and the cleaning costs. The properties of these coatings are determined by their very low surface energy. For this reason, they are safe for use with food products and can be sprayed or wiped on to surfaces with no problems posed by possible migration.

8.5

Nanosensors in packaging

The expiry date of a product is estimated by taking into account the distribution and storage conditions (especially humidity and temperature) to which the product are normally exposed.104,105 These conditions, however, are not always adhered to during the chain distribution. This is especially worrying for products that require a cold chain. Furthermore, micropores or sealing defects in packaging systems can lead to an unexpectedly high level of oxygen exposure, which can result in undesirable changes. Nanotechnology allows the development of packaging systems which monitor the condition of packaged goods to provide information during

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transport and storage.106,107 In fact, nanosensors able to detect pathogens, temperature changes, leakages and so on can be incorporated into clear packaging films or other packaging materials.108–112 The properties of the product enclosed in the packaging or the environment in which it is kept can be continuously monitored by the manufacturer, retailer and consumer.113,114 Devices that detect temperature or humidity levels over time are already available on the market. For example, stickers have been developed that rely on chemical reactions to provide consumers with more information about the quality of the product. These stickers change color depending on the duration and temperature of the product storage.108,112 Digital temperature data loggers measure and record the temperature history of shipments.15 The data can be downloaded (cable, radio-frequency identification (RFID), etc.) to a computer for further analysis. These help to identify if there has been temperature abuse of products and can also assist in determining the remaining shelf life.110 They can also help to judge when and for how long temperature extremes were experienced during shipment so that corrective measures can be taken. Nanotechnology offers new and more sophisticated tools to extend these capabilities and to reduce costs.116,117 When integrated into packaging, nanosensors can detect certain chemical compounds, pathogens, and toxins, helping to eliminate the need for inaccurate expiry dates and providing a real-time status of product freshness.118 A number of studies have described detection methods for toxins, allergens, bacteria and viruses that rely on nanotechnology. Such sensors might be able to detect and quantify spoilage and indicator organisms in packaged products and convey this information to those involved in managing supply chains information. These data can then be used to ensure the safety and quality of food delivered to commercial purchasers and ultimately to consumers. The possibility of combining biology and nanoscale technology into sensors offers the potential of increased sensitivity and therefore a significantly reduced response time in dealing with potential problems.119 Using the technologies currently available, testing for microbial contamination takes some days and the sensors that have been developed to date are too big to be transported easily. A bio-analytical nanosensor could detect a single virus particle long before the virus multiplies and long before symptoms are evident.120,121 This method offers several advantages including rapid and high-throughput detection, simplicity and cost-effectiveness, reduced power requirements and easier recycling. Carbon nanotubes coated with DNA strands have been employed as nanosensors for pathogen odors and tastes. The carbon nanotube functions as the transmitter while the strand of DNA functions as the sensor.122

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8.5.1 Radio-frequency identification (RFID) A RFID tag is a small, wireless integrated-circuit chip with a radio circuit and an identification code embedded in it.123 Much more than a barcode, a RFID allows the storage of data about the shelf life of the pack. The advantages of the RFID tag over barcodes are that it can hold much more information and can be scanned at a distance (and through materials, such as boxes or other packaging).124,125 RFID tags could be used on packaging to perform relatively straightforward tasks, such as allowing registration of hundreds of tags in a second or alerting the consumer if products have reached their expiry dates.126,127 RFIDs could be used to detect the condition of the packaging with the advantage that the information about the product could be electronically transferred from the product to devices in the logistical system. Currently, RFID technology still requires a silicon chip as a substrate for the high frequency electronics, but in a few years a low cost RFID will be available for use with ordinary packaging.128,129,2 Unlike earlier RFID tags, nano-enabled RFID tags are much smaller, can be flexible and are printed on thin labels.130,131 This increases the versatility of the tags (for example by enabling the use of labels which are effectively invisible) and thus facilitates much cheaper production. The tagging of packages will make it possible to monitor a product from the producer to the consumer, avoiding any type of counterfeiting.132–134 RFID tags are controversial because they can transmit information even after a product leaves the market. Privacy advocates are concerned that marketers will have even greater access to data on consumer behavior.

8.6

Packaging as a drug carrier and for drug delivery

The controlled chemical release of active compounds enables packaging to interact with the product it contains. Packaging can release antioxidants, flavors, fragrances or nutraceuticals to extend its shelf life or to improve its taste or smell.135–137 With these systems it is possible to avoid any type of under- or over-dosing. The release of the active molecules can be controlled by different mechanisms, including the diffusion of the permeant and the reaction of the carrier with a suitable element present in the packaging. In any case the release occurs in response to a particular trigger event.138–140 Distinct from anti-microbial packaging, which incorporates the active molecules and does not allow their release, triggerdependent chemical release packaging is designed to release biocides in response to the growth of a microbial population, humidity or other changing conditions.141 Another important area of research concerns the encapsulation of flavors and odors that can be released in particular conditions.

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Encapsulation can mask flavors, and is therefore also used for delivering supplements, vitamins and other additives.142 One interesting example of this is the encapsulation of both omega-3 fatty acids and β-carotene. These are both important for human health and can be added directly to the food as nanocapsules. Capsules produced in this way can be designed to break open only when they have reached the stomach. It increases the shelf life of the two molecules and resolves the problem of the unpleasant taste of some additives.143

8.7

Nanotechnology solutions for the packaging waste problem

An important strategic issue facing the packaging industry is the political and public pressure over the environment, particularly in relation to concerns over packaging waste.144 The use of nanomaterials, in addition to addressing spoilage and flavor issues, also offers other benefits such as lighter weight and better recyclability (Fig. 8.8). Producers may be able to significantly reduce transportation and production costs by reducing the amount of material used to package items.145–147 Nanostructured composites are also very cost competitive as a means of strengthening bioplastics, as they allow these materials to be used instead of petrochemical-based

8.8 Examples of Biodegradable food packaging.

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polymers for packaging and carrier bags.148–153 Developing sustainable packaging that can compete effectively with traditional packaging is extremely challenging.154–156 Increasing the natural materials used to produce waterresistant packaging will cause a major reduction in the amount of energy required to produce packages.157 Nanoclay particles, which will significantly improve the barrier properties and mechanical strength of the new biopolymer films and coatings, are probably the best candidates in this regard.158–161 Edible packaging, such as edible nanocoatings, can be used to make all kinds of foods spoilage-resistant, reducing the amount of packaging needed. A number of foods already come to market with a protective coat. Fruits and vegetables are often sprayed with wax to keep them from losing moisture. Edible coatings could be used to cover nuts to keep them fresh in packages or to keep them from going rancid in candy bars. Other uses include coating fragile foods such as breakfast cereals and sealing foods like salmon or sliced turkey, possibly with the addition of a natural anti-bacterial agent.

8.8

Anti-static packaging applications

Anti-static packaging is essential for most electronic products such as main boards, add-on cards and microchips.15 Electrostatic fields can lead to the formation of static charges that can damage these components.162,163 Antistatic packaging not only has to protect the enclosed objects from all forms of harmful static fields, but must also shield against moisture and be resistant to puncture or leakage.15,16 Anti-static packaging is made up of several parts and has a complex design. The packaging consists of a multilayer structure with a thin aluminum layer that ensures the conductibility of the packaging and contributes to its durability (Fig. 8.9a). Air bubbles can be wrapped in the packaging structure in order to absorb physical shocks. Anti-static bags are easier to manufacture directly from polymeric materials through the inclusion of various type of additives.164,165 They can be classified as migrating or permanent anti-static additives. Migrating additives diffuse into the polymer bulk, reaching up to the surface. Here they create a thin layer that attracts water molecules (Fig. 8.9b). The water molecules provide a conductive pathway that prevents the buildup of static electricity. Anti-static additives offer cost-effective protection for shortterm applications.166 For applications that require longer-term protection or lower surface resistivity, it is necessary to use permanent anti-statics or conductive additives.167 A conductive polymer can be added to the non-conductive polymer matrix in order to obtain a permanent static dissipation which is almost

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Hydrophilic group Moisture

Primer coating Polymeric core layer

Polymer matrix Metallic thin layer Low temperature sealable coating

Hydrophobic group

(a)

(b)

8.9 Examples of antistatic packaging: (a) multilayer structure; (b) chemical anti-static agent.

independent of the relative humidity. Other anti-static applications require the use of conductive particles and fibers such as carbon blacks and conductive fibers, including graphite and metals, which can be compounded into polymers to make them conductive. Graphite particles are used mainly in applications that require both thermal and electrical conductivity.168,169 For electronic applications that require minimal contamination, high integration density and uniform electrostatic dissipation in complex geometry, the standard conductive particles must be substituted with new conductive nanomaterials. Carbon nanotubes and carbon nanotubes are replacing carbon blacks and metallic particles in many electronic applications.170 carbon nanotubes are used at very low loadings that reduce weight and maintain or sometimes enhance the physical properties of the polymer matrix.171,172 Graphene sheets, which are essentially ‘unrolled’ carbon nanotubes, are one of the most conductive materials available.173–175 Graphene has a very high surface area (700–1000 m2/g) even compared to carbon nanotubes (300–500 m2/g) and expanded graphite (20–100 m2/g).176 Because of its large surface area, graphene improves mechanical properties even under extreme temperatures.174

8.9

Regulation and ethical issues in the new packaging industry

The rapid adoption of nano-based packaging in a wide range of consumer products has also raised a number of safety, environmental, ethical, policy and regulatory issues.177 The main concerns stem from the lack of knowledge concerning the interactions of nano-sized materials at the molecular or physiological levels and their potential effects and impacts on consumer health and the environment.178 Chemical-release packaging technologies are being designed to release flavors, odors or nutritional additives. Such

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packaging offers many advantages that should, however, be balanced against the potential new health risks associated with the use of nanomaterials.179–182 The preliminary results of a global study indicate that the migration of nanomaterials from the polymer nanostructured composites is generally minimal.183 However, scientific understanding in this regard can be considered very limited, and the issue is further confused by a number of experiments that have no bearing on real situations. For example, despite the wide application of nanosilver and many related studies on cytotoxicity to bacteria, there is still a serious lack of information concerning their longterm impact on human health and the environment. A recent study has found that silver nanoparticles can compromise DNA replication fidelity both in vitro and in vivo.184,185 It is clear, then, that more accurate research within the specific packaging area is essential. The potential toxicity and biological compatibility of nanomaterials need to be investigated and the potential hazards identified before these materials are widely incorporated into new and existing packaging applications.

8.10

Future trends

In the next few years, nanotechnology promises to develop a new concept of packaging following current trends including conservation of the environment, consumer demand and information technology. The environmental problems have been highlighted on a global scale, and a reduction in environmental loads is a must. A special effort must be undertaken to obtain a substantial decrease in the energy necessary for product distribution and to reduce the packaging materials required. The packaging of the future will therefore be designed to contain the minimum of materials and at the same time to offer maximum protection during transportation. Such packages require smart production technologies, transportation and testing to ensure optimal control along the entire distribution chain. Smart packages with indicator functions, providing detailed information about the packaging conditions and the distribution routes, will became indispensable. DNA-based biochips will detect the presence of harmful bacteria, while RFID will ensure a quick and accurate distribution of a wide variety of goods with limited shelf life. With an increasing number of consumer and industrial products containing engineered nanoparticles, however, more accurate research is necessary on the possible risks to humans and the environment. Several of these technologies are already on the market, while many others are currently being studied and developed. In a few years, nanotechnology will allow the packaging industry to provide a more satisfactory response to the demand for continuous enhancement of storage conditions, safety, convenience and information.

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9 Advanced protective coatings for aeronautical applications M. G. S. FERREIRA, M. L. ZHELUDKEVICH and J. TEDIM, University of Aveiro, Portugal

Abstract: This chapter deals with the issue of corrosion protection in aerospace structures. The types and factors which influence corrosion are reviewed, as are the protective coatings currently in use and those which have shown potential for future applications. The final sections focus particularly on functional nanocoatings for sensing corrosion, nanostructured coatings which self-heal when either corrosion begins or the corrosivity of the environment becomes critical, and other coating properties important for reducing maintenance costs. Key words: corrosion protection, nanocoatings, aerospace.

9.1

Introduction: corrosion in aeronautical structures

Materials used in the construction of aircrafts are chosen on the basis of their mechanical properties with the aim of achieving the highest possible aerodynamic efficiency. The strength-to-weight ratio is a decisive factor in the selection of materials. However, the corrosion-resistant properties of the selected materials are also of great importance, since metal corrosion is one of the greatest threats to the structural integrity of an aircraft and its functional systems. Metal corrosion is the oxidative degradation of metallic materials caused by various environmental factors in presence of oxidative agents such as oxygen, acids, and exhaust gases. Aqueous solutions containing dissolved atmospheric oxygen and different salts are the main source of corrosion in aircraft. The corrosion can affect the internal structure of the material or just cause superficial degradation. It can even lead eventually to structure failure if adequate inspection and maintenance procedures are not followed. Structures used in marine or offshore environments, or in areas with high levels of corrosive industrial fumes, are the most prone to corrosive attacks. The main corrosion processes of the metallic materials used in aeronautics are electrochemically driven. The local difference of the electrochemical potentials on the metal’s surface creates the driving force for the 235 © Woodhead Publishing Limited, 2011

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electrochemical redox reactions. Oxidation of the metal occurs on the anodic sites whereas on the cathodic zones the oxidative species are reduced. The cathodic and anodic processes can be separated in space by significant distances in the case of immersed structures or can occur in almost the same locations when exposed to strong caustic liquids. Many factors which can determine the rate of the corrosion processes should be taken into account when designing the aircraft. The design is based on the awareness of factors which cause corrosion and the types and mechanisms of corrosion that occur in aircraft structures. When designing airplanes to be resistant against corrosion degradation, the following factors should be considered: the selection of appropriate materials and their combinations; the application of adequate surface protection including corrosion inhibitors and sealants; the use of drainage to avoid excess condensation; and finally the maintenance of the aircraft during its service life. All these activities are essential to ensure control of corrosion on a predictable level and to guarantee the ability of the aircraft to carry its planned loads. The extensive use of composites in the airframes does not completely eliminate corrosion-related issues. While the airframe itself may not be subject to corrosion, the metal-made components are still prone to corrosion attack, and sometimes the combination of conductive composite materials and metals can even accelerate the corrosion attack because of the galvanic effects.

9.2

Types of corrosion in aircraft

Corrosion is a complex phenomenon which can take many different forms. These forms are influenced by a variety of factors, such as environmental conditions, type of corrosive agents present, the metal involved and its geometry as well as mechanical loads. Some forms (corrosion fatigue or stress corrosion cracking) of corrosion can propagate very quickly (days or even hours) causing catastrophic structural failure that occurs without warning. The most common types of corrosion found on airframe structures are briefly described below. One of the most visible types of aircraft corrosion is surface corrosion. This type of corrosion is typical on aluminum and magnesium alloys. The surface corrosion can propagate on both uncoated and coated metallic surfaces. On bare metals this type of corrosion appears as etching or general roughening with a formation of powdery deposits of corrosion products. Very often surface corrosion spreads under the coating and cannot be easily recognized. The undermining and creepage processes lead to the coating disbonding and to the loss of its barrier properties. An occluded environment (Lewis et al., 1999) can be formed under the coating, leading to the

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autocatalytic propagation of the corrosion process (Schneider and Kelly, 2007). Another form of surface degradation is filiform corrosion. It can be recognized by its characteristic worm-like trace under the polymer protective coating (Fig. 9.1). The filiform attack normally occurs in slightly acidic conditions and at a relatively high humidity (80–90%). Filiform corrosion on aluminum alloys is a complex process influenced not only by

2 mm

2 mm Corrosion Individual front filaments (a)

(b)

1 mm (c) 1 mm

(d)

9.1 Optical micrograph of the scribe and filaments that have grown from the scribe under a polyurethane topcoat for (a) and (c) alkalinecleaned and (b) and (d) chromate conversion coated AA2024-T351 aluminum alloy. (Mol et al., 2004)

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the environment, but also by the properties of applied polymer coatings (Romano et al., 2009) and the adhesion on the metal/polymer interface. The effect of different parameters is interdependent and cannot be easily separated. The microstructure of aluminum alloys plays an important role in the filiform corrosion susceptibility (Mol et al., 2004). The metal/coating interfacial adhesion under wet conditions has, however, been considered to be the decisive factor in controlling filiform corrosion attack (Fedrizzi et al., 1998). Crevice corrosion is also driven by the differential concentration mechanism. It is the most common type found on airplanes. The crevice attack occurs whenever water is trapped between two surfaces such as unsealed joints or delaminated bond-lines. Areas where fasteners are fixed are also weak points as shown in Fig. 9.2 (Matarredona et al., 2003). In many cases, crevice corrosion can quickly initiate other more dangerous types of corrosive attack such as pitting and exfoliation corrosion. Advanced crevice attack causes skin buckling and eventual spot weld fracture. The most effective strategy to prevent this kind of corrosion is to avoid water penetration to the joint by applying sealants or by surface hydrophobization (Matarredona et al., 2003). One of the main driving forces of pitting corrosion is a concentration difference effect. Pitting corrosion results in a localized loss of material due to a corrosive attack confined in an extremely small area. Although only a very little amount of metal is removed, the pits can act as stress risers that lead to fatigue failure if located in critical load zones. Stainless steel and aluminum and magnesium alloys used in the aeronautical industry are especially susceptible to this type of corrosive attack. In the case of stainless steel, the initiation of pitting begins in localized areas where the passive oxide film has breaches exposing the bare metal to the environment. These

Water Fastener

Al

Al

9.2 Fastener joint. (Adapted fram Matarredona et al., 2003)

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active spots become anodes while the rest of the passive surface functions as a large cathode. This triggers rapid metal dissolution in the small active zones as a result of the high cathode/anode surface ratio. Pitting corrosion in aluminum alloys usually originates from the electrochemical nonuniformity of the surface and related galvanic effects. The cathodic intermetallics are often the starting points which lead to a localized dissolution of the adjacent aluminum matrix. Aluminum alloys are also susceptible to intergranular corrosion which occurs along the grain boundaries of an alloy and is generally the result of a lack of uniformity in the alloy structure (Huang and Frankel, 2007). Intergranular corrosion is difficult to detect in the early stages since it may exist without visible surface evidence. Very severe intergranular corrosion may sometimes cause lifting or flaking of the metal due to the delamination of the grain boundaries. The buildup of corrosion products creates pressure between the metal grains and causes exfoliation. Exfoliation occurs most frequently in wrought products, such as extrusions, thin plate, thick sheet, and certain die-forged shapes, which have a thin, highly elongated platelet type grain structure (FAA, 1991). The airframe of the aircraft is made up of different metallic and composite materials. This combination of different electrically connected metals in the presence of electrolytes can result in the formation of electrochemical galvanic couples and lead to galvanic corrosion. Extensive localized damage may result from contact between dissimilar metal parts. The rate of galvanic corrosion also depends on the geometry of the multimaterial assembly. The high ratio of the surface area of the cathode to the surface area of the anode will lead to a swift and severe localized attack. When the surface of the active metal is larger than that of nobler metal the corrosion will be slower and more superficial. For example, an aluminum fastener in contact with a relatively inert Monel structure may corrode severely, while a Monel bracket secured to a large aluminum part would result in a relatively low superficial attack on the aluminum sheet (FAA, 1991). Galvanic corrosion can be very serious because in many instances it occurs out of visible zones. The only way to detect it prior to structural failure is by disassembly and inspection (FAA, 2008). Galvanic corrosion can also occur when carbonfibre reinforced plastics are joined with metallic components. The composite acts as an effective cathode and accelerates the electrochemical dissolution of the adjacent metal. Graphite fibers, which are used as reinforcing elements in composites, present a particularly challenging galvanic corrosion combination. These fibers are highly electrically conductive and greatly increase the risk of galvanic corrosion in the aluminum alloys used in the aircraft structure. The only effective method of avoiding corrosion in this case is to prevent moisture from simultaneously contacting the aluminum structure and the

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composite by ensuring a physical barrier between them using a coating or a sealant, and by providing drainage. Figure 9.3 shows the Boeing 777 carbon-fiber reinforced plastic (CFRP) floor beam design and corrosionprotection methods. An aluminum splice channel is used to avoid attaching the floor beam directly to the primary structural frame (Nichols, 1999). The improper use of steel cleaning products, such as a steel wire brush on aluminum or magnesium, can also induce dissimilar metal corrosion because of the small steel debris left on the surface. The impact of corrosion on aircraft structures is often magnified by mechanical factors such as residual stresses or the service loads. Areas

Detail area Sealed CFRP cut edges Fasteners installed with sealant Fay seal

CFRP floor beam with cocured fiberglass ply Aluminum splice channel Aluminum frame

9.3 Boeing 777 design incorporating CFRP. (Nichols, 1999)

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under mechanical stress degrade at an accelerated rate compared to the one they would deteriorate at if only subjected to corrosion. Examples of mechanically accelerated corrosion damage in aircrafts are fretting corrosion, corrosion fatigue, and stress-corrosion cracking. Fretting corrosion occurs at the interface between two loaded surfaces which move against one another because of different mechanical vibrations. The passive film is removed from the metal’s surface by the rubbing action causing severe localized corrosion. Stress-corrosion cracking (SCC), also known as environmentally-assisted stress-corrosion, is a cracking of the metal caused by a combination of mechanical stress and corrosive impact. This type of corrosion can occur in two modes, intergranular stress-corrosion cracking, which is the more common form, or transgranular corrosion. SCC of the most relevant aluminum alloys is characteristically intergranular and appears where the conditions along grain boundaries make them anodic in respect to the grains so that corrosion can propagate selectively along them. This type of corrosion can quickly cause a loss of load-carrying capability. Certain environments cause SCC of specific alloys. Salt solutions and seawater, for example, can cause SCC in high strength heat-treated steels and aluminum alloys; methyl alcohol–hydrochloric acid solution has an impact on some titanium alloys whilst magnesium alloys are susceptible to SCC even in moist air. SCC can be reduced by applying protective coatings, or corrosion inhibitors or by controlling the environment. An alternative approach is to use specific heat treatments or shot peening. Corrosion fatigue is another form of mechanically accelerated degradation and is caused by the combined effects of cyclic stresses and corrosion. The damage from corrosion fatigue is stronger than that of both cyclic loads and corrosion put together (FAA, 1991). Fracture of a metal part caused by corrosion fatigue occurs at stress levels far below the fatigue limit in air. Therefore protection of all the aircraft parts subjected to alternating stresses is especially important even in environments which are only mildly corrosive.

9.3

Factors influencing corrosion

Many factors influence the type, rate, and seriousness of metal corrosion. Some of these can be controlled in the case of aircraft and some, like the climate, cannot. The environmental conditions under which an aircraft is operated strongly affect the corrosion processes. In a marine environment (with exposure to sea water and salt air), the moisture-laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate. Temperature is also an important factor since the rate of electrochemical attack is increased in a warm,

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humid climate. Amongst the controllable factors which influence the rate of corrosion is foreign material which contacts and adheres to the metal surfaces. Such materials include: salt water and salt moisture condensation; soil and atmospheric dust; oil, grease, and engine exhaust residues; spilled battery acids and caustic cleaning; welding and brazing flux residues. It is very important to keep aircraft clean. The periodicity and method of cleaning depend on the exploitation conditions, model of aircraft, and type of operation. The natural and technical environments which metallic aircraft structures come into contact with have different levels of corrosiveness and therefore cause different types of corrosion. The most important atmospheric corrosive agents are oxygen and water vapor. This combination can quickly cause corrosion degradation. The atmosphere very often additionally contains corrosive gases and contaminants, especially in marine and industrial zones, which can be extremely corrosive to the metallic components of an aircraft. There is a manifold increase in the aggressiveness of the moisture when salts are dissolved in it. The exposure of airframe metallic materials to salt solutions is extremely undesirable. The most corrosive anion for aircraft metals is chloride. Some stainless steels are resistant to salt, but other steels, as well as magnesium and aluminim alloys, are extremely sensitive. Even moderately strong acids can significantly corrode most of the alloys used in an aircraft. The most corrosive are sulphuric acid (batteries), hydrochloric acid, NOx compounds, and the organic acids found in human waste. Alkalis are generally not as corrosive as acids. For example, aluminum alloys are highly resistant to ammonia, but some alkaline solutions can cause severe corrosive damage to both magnesium and aluminum alloys. Particularly corrosive to aluminum alloys are washing soda, potash, and lime. An additional acceleration factor is the presence of microorganisms which can change the local environment and increase its corrosiveness. This problem is especially important when considering the corrosion processes in the fuel tanks of aircrafts. The microbial growth of fungi occurs at the interface between water and fuel where the fungus feeds off fuel producing organic acids, alcohols, and esters. The fungus normally attaches itself to the bottom of the tank and has the appearance of a brown deposit (FAA, 1991). A lot of effort is devoted to increasing the corrosion resistance of aircraft by improving the employed materials, surface protection, insulation, and design of components. All these efforts aim to increase the aircraft’s reliability and reduce the maintenance costs. Despite these improvements, aircraft still require continuous preventive maintenance. The prompt detection and removal of corrosion will limit the extent of degradation in aircraft components (FAA, 1991; FAA, 2008). The basic philosophy of a corrosion prevention program consists of the following:

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Table 9.1 Susceptibility of different metallic materials to corrosion attack Alloy

Type of attack to which alloy is susceptible

Aluminum

Surface pitting, intergranular, exfoliation, fretting, stress-corrosion cracking, corrosion fatigue Highly corrosion resistant, extended or repeated contact with chlorinated solvents may result in degradation of structural properties at high temperature Highly susceptible to pitting Surface oxidation and pitting, intergranular Crevice corrosion, some pitting in marine environments, corrosion cracking

Titanium

Magnesium Low alloy steel Stainless Steels

Source: FAA (1991)

• • • • • • • • •

detailed inspection for corrosion and failure of protective systems on a scheduled basis; early detection and touch-up of damaged paint areas; aircraft cleaning at regularly scheduled intervals; thorough periodic lubrication; keeping drain holes open and functional; daily wiping of exposed critical areas; inspection, removal, and reapplication of preservation compounds on a scheduled basis; sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days; maximum use of protective covers on parked aircraft.

Table 9.1 summarizes information on the susceptibility of main metallic materials to different types of corrosion.

9.4

Corrosion of aluminum and its alloys

Aluminum is still one of the most widely used materials in modern aircraft construction. It is vital to the aeronautical industry because of its high strength-to-weight ratio (Starke and Staley, 1996). Commercially pure aluminum has a tensile strength of about 13 000 psi, but, by alloying it with other metals and using the appropriate heat treatment, the tensile strength can be increased to as high as 65 000 psi which is within the strength range of structural steel. The various types of aluminum can be divided into two main classes: wrought alloys and casting alloys. Wrought alloys are the most widely applied in airplane design and are used for stringers, skin, rivets, bulkheads, and extruded sections. The total proportion of alloying elements

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is normally not above 6 or 7% in wrought alloys. Aluminum alloys in which the principal alloying elements are manganese, chromium, or magnesium and silicon demonstrate relatively high corrosion resistance in different environments. Alloys containing substantial percentages of copper, however, are more prone to corrosive degradation (FAA, 2008). The general corrosion of aluminum usually develops very slowly, but it can be accelerated by the presence of dissolved salts and other corrosive agents. The more dangerous types of corrosion are different localized forms such as penetrating pit-type corrosion (Yasakau et al., 2006), SCC (Scamans et al., 2010), and intergranular corrosion (Huang, 2005). Corrosion-resistant aluminum alloys and tempers are used to increase resistance to intergranular corrosion and SCC. An example of such a change is the replacement of 7150-T651 aluminum plate on upper wing skins with 7055-T7751 plate, which is not as susceptible to localized corrosion. Normally the major structural forgings are shot peened in order to improve the fatigue life of aluminum parts and to reduce susceptibility to SCC (Nichols, 1999). Pure aluminum is considerably more resistant to corrosion than higher aluminum alloys. A thin layer of relatively pure aluminum (commonly called ‘Alclad’) is therefore often applied over the base aluminum alloy. The Alclad layer makes the surface composition more uniform, consequently reducing the susceptibility of the alloy to localised corrosion. An additional important function of the aluminum layer is its sacrificial galvanic corrosion protection. This is possible because pure aluminum is more electrochemically active than the alloy matrix. In cleaning such surfaces, however, care must be taken to prevent staining and marring of the exposed aluminum and, more importantly from a protection standpoint, to avoid unnecessary mechanical removal of the protective Alclad layer and the exposure of the more susceptible aluminum alloy base material (FAA, 2008). Clad aluminum materials are used where weight and function permit, such as for fuselage skins. More detailed approaches to the protection of aluminum based aircraft materials using different surface treatment technologies are presented in the following sections.

9.5

Corrosion of magnesium alloys

Magnesium is the lightest structural metal available on our planet. Its specific gravity is 1.74 (the SG of aluminum is 2.7 and that of steel is 7.9 on average) (Ostrovsky and Henn, 2007). It is, therefore, highly useful for building structures which require high mechanical properties at a low weight. Some magnesium-based alloys and especially magnesium wrought materials could be effective alternatives to aluminum alloys because of their low weight, good mechanical properties, and metallic behavior.

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Magnesium alloys have been used in airplanes since the 1930s. In the 1950s, there was a boom in the use of magnesium during which it was widely applied in aircraft structures. The aeronautical industry of the former Soviet Union especially broadly used magnesium in military and civil aircrafts. For example, the Tupolev TU-134 had 1325 magnesium components with total weight of 780 kg. Among the aircraft parts that have been made from magnesium with substantial savings in weight are nosewheel doors, flap cover skin, aileron cover skin, oil tanks, floorings, fuselage parts, wingtips, engine nacelles, instrument panels, radio masts, hydraulic fluid tanks, oxygen bottle cases, ducts, and seats (FAA, 1991). However, Airbus, Boeing and Embraer have not extensively used magnesium in their structural applications for airplanes until now. The situation is different in the helicopter industry where magnesium is used in cast gearboxes and transmissions as well as some other non-structural elements. There are two main reasons which justify the relatively low use of magnesium alloys in aircraft. First, there has been a lack of high strength magnesium alloys for structural applications. However, recently Magnesium Elektron Ltd (UK) has developed new high strength alloys Elektron® 21 and Elektron® 675 which have mechanical properties comparable to aerospace aluminum structural alloys. The second limiting factor is more important and is related to the corrosion problem. Magnesium is the most chemically active metal used in aircraft construction. This means that magnesium alloys have a very high susceptibility to different types of corrosion even in not very aggressive environments. Magnesium corrosion is probably the easiest type of corrosion to detect in its early stages since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed. The first signs of this type of corrosion are a lifting of paint films and white spots on the magnesium surface. These rapidly develop into snow-like mounds or even ‘white whiskers’. Re-protection involves the removal of corrosion products, the partial restoration of surface coatings by chemical treatment, and the reapplication of protective coatings. Magnesium castings, in general, are more prone to penetrating attack than wrought magnesium skins. Correctly surface treated, painted, and insulated magnesium skin surfaces show relatively high corrosion resistance if the original surface is maintained. Drilling and riveting can damage the original surface treatment, however, and it may not be completely restored by touch-up procedures. The paint thickness is also usually significantly thinner at the edges raising a potential corrosion problem whenever magnesium is used. Corrosion inspection should include all magnesium surfaces with special attention to edges, fasteners, and cracked or chipped paint. Development of effective new protection technologies for magnesium alloys is therefore of primary importance and can drastically increase the application of magnesium-based materials in the aeronautic industry.

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9.6

Protective coatings in aerospace engineering

Although reviews on this subject can be found in the literature (Cohen, 1995; Twite and Bierwagen, 1998) the authors have tried, in the present section, to condense the main points concerning protective coatings and pre-treatments in use or which have the potential to be used in aeronautics. Nanocoatings are not included in this section and will be discussed in more detail in Sections 9.9–9.12. The aerospace industry places a high demand on the coatings used to paint or repaint the existing structures. Each structure relies on several layers of coatings to provide improved adhesion, protection against the environment and corrosion, visual aesthetics, and other specialized functions (Twite and Bierwagen, 1998). A typical coating system comprises three individual coating layers (Fig. 9.4). The first layer is a conversion coating and is usually a very thin (<10–60 nm) inorganic layer providing corrosion protection and improved adhesion between the substrate and the primer, which is the second layer of the coating system. Currently, a chromate conversion coating is used. The primer comprises a pigmented organic resin matrix. The application thickness of the primer can vary from 5 to 200 μm although a thickness of 25 μm is usual in the case of aircraft due to weight constraints. The primer is the principal provider of corrosion protection. Typical formulations consist of both chromated and non-chromated pigments enveloped in an epoxy resin. Currently, in order to obtain high corrosion protection strontium chromate or zinc chromate pigments are used (Twite and Bierwagen, 1998). An organic coating alone is not sufficient to protect an underlying metal substrate from corrosion. In general, a coating contains micro-pores, areas

Presently a flexible polyurethane/ polyol or polyester Presently an epoxy-polyamide with Sr chromate Presently Alodine 1200 with chromate Al 2024 T3

Topcoat

Primer Pre-treatment Substrate

Topcoat appearance/ survivability Primer corrosion protection Pre-treatment adhesion/ corrosion protection Substrate structural stability

9.4 Schematic of current aerospace coating system. (Adapted from Bierwagen et al., 2010; Twite and Bierwagen, 1998)

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of low crosslink density or high pigment volume concentration (PVC), which provides a path diffusion of corrosive agents, such as water, oxygen, and chloride ions, to the coating/metal interface. It is most often necessary, therefore, to incorporate inorganic or organic inhibitors into a paint system for corrosion protection. The most effective corrosion inhibitor that has been used so far is hexavalent chromium (Cr6+). After the primer, a top coat is applied, which serves as the main barrier against environmental influences such as extreme climates and ultra-violet rays. It also provides the aircraft with decoration and in some cases camouflage. A typical top coat is formulated using a polyurethane or polyester resin and the application thickness varies from 50 to 200 μm. A complete coating system consists of the pre-treatment, primer and topcoat. Although the traditional coatings for aluminum alloys used in the aerospace industry are based on hexavalent chromium, the use of chromates and other chromium containing products is highly regulated due to their carcinogenic effects and lack of environmental safety (Tiley, 1992; Xianglin and Dalal, 1994; USDA, 1995). Many attempts have been made to replace chromate-based pre-treatments (Kendig and Buchheit, 2003), some of which are discussed in this chapter.

9.7

Pre-treatments

9.7.1 Chromate conversion coatings Chromates have been around since the early twentieth century as a means of controlling the corrosion of active metals (Bucheit, 1995). Their use, however, has led to progressively greater restrictions, imposed by national and international legislation, relating to concerns over health, safety, and environmental protection, regarding the use of these treatments. New, nonhexavalent chromium-based processes are therefore becoming commercially available (Battelle, 2000) and the aeronautics industry has become a major chromates consumer until a viable alternative can be found. Chromate conversion coatings are obtained through a chemical reaction with the metal being treated which forms a complex chromate film over the entire surface. Chromate conversion is carried out by immersion, brush application, or spraying. The process is named after the chromate found in the chromic acid used in the bath, more commonly known as hexavalent chromium. IriditeTM and Alodine® are brand names for chromate conversion coatings that belong to MacDermid Inc and Henkel, respectively. Some important standards for chromate conversion coating on aluminum are MIL-DTL-5541 (COMA, 2006) and ASTM B449-93 (COMA, 2004). The composition of chromate conversion solutions varies widely depending

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on the material to be coated and the desired effect. Most solution compositions are proprietary. Chromate coatings are soft and gelatinous when first applied, but harden and become hydrophobic as they age (Osborne et al., 2001). Curing can be accelerated by heating them up to 70 °C, but higher temperatures will gradually damage the coating over time. Some chromate conversion processes use brief degassing treatments at temperatures of up to 200 °C, to prevent hydrogen embrittlement of the substrate. Coating thicknesses vary from a few nanometers to a few micrometers (Edwards, 1997). The chromate coatings are suitable for use on all types of aluminum alloys including high silicon pressure die-castings, which are difficult to anodize. In addition, they can be used to treat zinc and magnesium alloys to promote greater paint adhesion. Chromates also have low electrical contact impedance. Chromates are used for their strong oxidizing powers, their solubility in water, the passive nature of their reduction products, and their cost and ease of application (Kendig et al., 1995). The exact mechanism for corrosion inhibition by Cr6+ is still being debated, but it is accepted that the solubility of Cr6+ in water aids in its transport to actively corroding sites where it passivates the area by reducing to Cr3+ (Kendig et al., 1993). Chromate treatments produce effective paint bonds through their molecular adhesion with the film bound to the metal and to the organic coating. The film lengthens the life of the paint, forming an effective barrier against corrosion caused by pores or scratches in the paint.

9.7.2 Chromium-free inhibitors The desired attributes of any new inhibitors are: a high capacity in the coating, a high solubility in the aqueous phase over the coating when released, and high inhibiting efficiency. Among the most promising candidates being investigated as active inhibitors are rare earth (RE) compounds, molybdates, vanadates, phosphates, and certain organic corrosion inhibitors. The salts of RE elements were found to provide an effective corrosion inhibition effect to aluminum alloys (Bethencourt et al., 1998; Twite and Bierwagen, 1998; Davo and Damborenea, 2004). They control the cathodic reaction by precipitating metal hydroxide (Ln(OH)3) at local regions, which is associated with an increase of pH due to oxygen reduction (Aballe et al., 2001; Arenas et al., 2001; Davo and Damborenea, 2004). Cerium showed maximum corrosion protection efficiency compared with other RE compounds (Yasakau et al., 2006). Cerium nitrate revealed superior corrosion inhibition properties in comparison with lanthanum nitrate, probably due to the lower solubility of the hydroxide. The insoluble film at

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the cathodic sites blocks the cathodic reduction of oxygen. As a result, the current supplied to the anodic reaction decreases and the aluminum dissolution is reduced. An important role in the superior efficiency of cerium can be also played by Ce4+, which can be formed at high pH values in aerated chloride environments (Bilal and Muller, 1992; Aldykiewicz et al., 1996). RE compounds can be introduced in corrosion protection systems for aluminum alloys using different strategies. The formation of a conversion coating composed of a hydrated oxide layer on top of the aluminum alloy offers an enhanced corrosion protection (Campestrini et al., 2004; Palanivel et al., 2005). Another approach is the use of the cerium conversion coating technique to seal the porous film of the anodized aluminum alloy (Yu and Cao, 2003). The cerium-based inhibitors can also be introduced in the thin hybrid coatings used as pre-treatment for aluminum alloys where they have exhibited promising results (Kasten et al., 2001; Voevodin et al., 2001a; Zheludkevich et al., 2005a). The introduction of cerium compounds can, however, decrease the stability of the hybrid polymer matrix by decreasing the barrier properties. The introduction of zirconia nanoparticles doped with cerium ions into a hybrid sol−gel matrix was found to successfully avoid the negative effect of the cerium cations on the film and to provide a prolonged release of cerium inhibitor in the places of localized corrosion (Zheludkevich et al., 2005a,b). Although many works have been dedicated to the investigation of corrosion inhibition using RE-based inhibitors, there are still numerous contradictions and ambiguities concerning the mechanism of this inhibition. Molybdates are corrosion inhibitors and their capabilities were attributed to the formation of precipitates of molybdenum oxide or hydroxide species atop the S-phase on the AA2024 aluminum alloy. The formation of precipitates is thermodynamically possible via the reduction reaction of molybdate anion to insoluble molybdenum (IV) oxide (Yasakau, 2011). Vanadates are good corrosion inhibitors if used in the form of monovanadates. This species inhibits the oxygen reduction reaction to a level similar to chromate. Decavanadates, however, are detrimental or ineffective inhibitors (Iannuzzi et al., 2006). Organic inhibitors can be adsorbed on the metal surface or can form strong complexes with copper or other constituents of the alloy, effectively reducing the activity of AA2024. The inhibiting action is based on the passivation of active intermetallic zones, which prevents the dissolution of magnesium and aluminum as well as the dissolution and redeposition of copper. The insoluble layer also prevents the adsorption of aggressive chloride ions on the surface of the alloy. Some examples of effective corrosion inhibitors, which confer long-term corrosion protection to the AA2024 alloy in neutral chloride solution are 8-hydroxyquinoline (8HQ), quinaldic acid

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(QA), 2-mercaptobenzothiazole, and benzotriazole (Zheludkevich et al., 2005c; Lamaka et al., 2007).

9.7.3 Sol–gel coatings Sol–gel processes were initially developed for the deposition of inorganic oxide coatings, but in recent years, the synthesis of organic–inorganic hybrid coatings has acquired great importance (Fig. 9.5). These hybrid films have attracted interest because they combine the properties of the organic polymeric materials and those of the ceramics. Hybrid coatings can be prepared over a continuous compositional range from almost organic to almost inorganic (Metroke et al., 2001). Their properties can be changed continuously to form an optimum coating (Mackenzie and Bescher, 2003). The inorganic components contribute to the increase of scratch resistance, durability, and adhesion to the metallic substrate. The organic components increase the density, flexibility, and functional compatibility with organic paint systems. An advantage of the organically modified composites is the formation of coatings which are thick and homogeneous (Whang et al., 2001). The use of organofunctional silanes can improve the mechanical properties and adhesion to specific organic paint systems in comparison to sol–gel materials based on non-functional organosilanes (Joshua et al., 2001; Hofacker et al., 2002). The commonly used sol–gel precursors are commercially available as organoalkoxysilanes and metal alkoxides. Controlling the hydrolysis and condensation of low molecular weight alkoxides during sol–gel processing is a possible way to produce the desired molecular or nanoscopic mixture. Many structures with different bonds between the two networks, ranging from covalent bonds to physical mixing, are dependent on the process conditions (Silva and Vasconcelos, 1999). The synthesis and relevant properties of hybrid materials have been extensively reviewed (Sanchez et al., 2001; Soler-Illia et al., 2002). Such hybrid networks are obtained through the hydrolysis of organically modified silicon alkox-

X3Si

X3Si

SiX3

(CH2)n

(CH2)n

Si

Si

Curing

(CH2)n O

Si

OH OH OH OH OH OH Hydroxide / Oxide AA 2024-T3

SiX3

Hydroxide / Oxide

H2O

(CH2)n O

Si

O

O

Me

Me

O

AA 2024-T3

9.5 Silane deposition on a metallic substrate. (Cabral et al., 2005)

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ides condensed with or without metallic alkoxides. These technologies are simple and yield amorphous hybrid nanocomposites, but may present a large size dispersion and heterogeneous chemical composition (Sanchez et al., 2003). The polymer network can be reinforced by dispersing particles in the hybrid matrix. Different strategies for nano-particle introduction can be used, such as the addition of nanopowders into the sol–gel system and the design of functional nanostructured materials through the use of controlled hybrid organic–inorganic interfaces (Palanivel et al., 2002, 2003; Roux et al., 2003; Sanchez et al., 2003). The characteristics of the hybrid sol–gel coatings make them suitable for pre-treatments of metallic substrate aimed at improving corrosion protection. The use of these pre-treatments for the corrosion protection of aluminum alloys is a recent, but promising approach. The sol–gel coatings confer enhanced corrosion protection properties due to the formation of a barrier against the penetration of the corrosive species. According to Yang et al. (2001), aluminum oxide and silicon oxide can form a stable oxide layer at their interface after the beginning of the corrosion process, which delays the onset of pitting corrosion. Voevodin et al. (2001b) also report that organically modified sol–gel coatings exhibit significantly better corrosion protection, in terms of barrier properties, than the typical chromate pre-treatment. The new trends in corrosion research focus on the use of sol–gel coatings modified with inorganic particles or fillers. These additions increase the corrosion protective properties of the hybrid sol–gel coatings. The concentration and size of the particles, however, may affect the corrosion resistance of the protective coating. Electrochemical impedance measurements indicate that the sol–gel film, heavily loaded with inorganic particles, can form a porous film, which increases electrolyte uptake into the system, causing coating delamination (Palanivel et al., 2002; Conde et al., 2003). It was found that the optimal particle/matrix loading by weight in such systems is approximately 20%. The corrosion protection properties are improved by the reduction of the particle size. Hybrid coatings with nanostructured oxide particles seem, therefore, to be very promising for the protection of metals and alloys from corrosion. Preliminary results show that hybrid coatings with nano-particles are effective in promoting the adhesion of organic paints to the substrate because they act as effective coupling agents. Additionally, the incorporation of inorganic nanoparticles can be a way to include inorganic salt corrosion inhibitors, preparing an inhibitor reservoir for ‘self-repair’ pre-treatments with controlled release properties (Joshua et al., 2001). Zheludkevich et al. (2006) studied the corrosion behavior of AA2024-T3 alloy pre-treated with sol–gel hybrid coatings, containing ZrO2 nanoparticles. They showed that the presence of these nanoparticles enhances

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the corrosion protection of the sol–gel hybrid coatings and may also lead to an important pore blocking effect. The corrosion behavior is dependent on the nature of the silane. With three different silanes, bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) provided higher corrosion resistance than bis-1, 2-[triethoxysilyl] ethane (BTSE) or 3-mercaptopropyltrimethoxysilane (MPS). The systems also showed good adhesion tests and the fatigue tests revealed that the silane films contributed to the enhanced fatigue life of AA2024-T3 compared to the bare substrate (Cabral et al., 2005). The Boegel (Boeing developmental name) sol–gel conversion coating consists of a dilute aqueous zirconium and functionalized silicon alkoxide solution that is spray applied to the cleaned metal surface. No rinsing is required and the film is dried in place under typical ambient laboratory conditions (Blohowiak et al., 1998; Osborne et al., 2001). The silicon component carries an organic group which is chosen for its chemically compatibility with the organic polymer system in the primer or topcoat. A glycidoxyl group is typically used for epoxy primers. The coating system is thin (typically 50–200 nm) and forms a gradient coating as shown schematically in Fig. 9.6. A key feature is that chemical bonding is possible between the sol–gel coating and both the substrate and the primer or topcoat. This conceptually gives superior adhesion relative to chromate conversion coatings which rely on mechanical interlocking, hydrogen bonding, and dispersion forces for adhesion (Osborne et al., 2001).

9.7.4 Magnesium-rich primers The fact that magnesium could offer cathodic protection for aluminum alloys in a manner similar to zinc metal pigments used in zinc-rich coatings Organic resin

Organic resin NH2

NH2

NH2 O

O

O

R′ Si

O

O O

O

R′

O

O

Si

O

O

O

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Si

Zr

Si

Zr

O

O Zr

R′

O Si

O

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Zr

Zr

O

O O

O

O O

Si O

Zr

Zr

O Zr

Part

Si

Si

O Si

O

NH2

O

O

O

Zr

O

NH2

Si

O

R′

O

O

Si O

O

O

O

Sol– gel

NH2

O

Zr

O O

R′

Metal part surface

9.6 Representative sol–gel coating structure. (Osborne et al., 2001)

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to provide cathodic corrosion protection for steel prompted Bierwagen (2008) to formulate magnesium-rich coatings for the chromate-free protection of aerospace alloys. The essential features of such a coating are that the magnesium metal particles would have to connect with themselves and the metal substrate, and thus be added at a PVC very near to the critical pigment volume concentration (CPVC), as is acknowledged for zinc-rich coatings, and the coating matrix would have to be a material able to withstand basic conditions (Bierwagen, 2008). If the coating was designed properly, the open circuit potential (OCP), also known as the free corrosion potential (Ecorr), of the mixed system would take a mixed potential value between the OCPs of the two metals involved in the system (Felix et al., 1993; Hare, 2000). This indeed proved to be the case for magnesium-rich coatings over AA2024-T3 and AA7075-T6, the two most commonly used aerospace alloys, and two of the most corrosion prone alloys of this class (Battocchi et al., 2006a; Bierwagen et al., 2007). Further studies gave a more complete description of the corrosion protection afforded by this new class of coatings (Bierwagen et al., 2010). As with zinc-rich coatings, the corrosion protection afforded by the magnesium-rich systems is driven by the metal pigment, and is only partially dependent upon the polymer for providing total corrosion protection. The total system performance of the magnesium-rich primers and topcoat is a synergistic blend of the cathodic/sacrificial protection of the primer, the inhibition/thin barrier layer effects of the MgO/Mg(OH)2 formed as oxidation products of the magnesium, the barrier properties of the polymer in the magnesium-rich primer, plus the barrier properties of the topcoat (Battocchi et al., 2006b; Hosking et al., 2007).

9.8

Anodizing coatings

Anodizing is a common process in aeronautical industry, and the electrolysis process consists of the formation of a strong and stable oxide film for aluminum and its alloys. Corrosion and mechanical (abrasion) properties are improved and the metal oxide is a good basis for an organic layer above. This is vital to achieve desirable levels of strength and reliable long-term performance and has been in use for decades. The most common anodizing processes for aluminum use chromic acid, sulfuric acid, or oxalic acid (Wernick et al., 1987). Other acids such as phosphoric acid and boric sulfuric acid mix are now used in the market for anodizing in the aerospace industry. All the processes use an electrical current to form the oxide film. The current passes through an electrolyte in which aluminum is the anode, hence, the name ‘anodizing.’ The nature of the electrolyte, the reaction produced and operation parameters determine the structure and properties of the formed oxide film.

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Chromic acid anodizing (CAA) is currently used in the aerospace industry in order to treat high strength aluminum alloys, such as AA2024-T3. However, the process uses chemicals which contain Cr VI, which are illadvised from both a health and an environmental point of view. The corrosion resistance is excellent relative to the thickness of the coating, which normally lies in the range of 2–5 μm. The oxide film is softer and less porous than those formed by the other processes, and is formed without any significant fatigue loss of the material (Juhl, 2010). The film is easily damaged, and the color is light opaque gray. When this film is sealed in a dichromate seal, a greenish color appears. A seal in a hot dilute chromate solution or hot water is required to achieve satisfactory corrosion resistance. Sulfuric acid anodizing (SAA), although used in the aeronautic industry, leads to a decrease in mechanical fatigue and thus the process is not allowed in high resistance structural parts (Domingues et al., 2003b). Oxalic acid is now mainly used in hard coat anodizing to produce a hard coating faster than that obtained using a pure sulfuric acid electrolyte. In the late 1990s, processes involving sulfuric boric electrolytic baths (SBA) were developed. Although the beneficial effects of borate to the corrosion resistance of AA2024 alloy have not been proven, the results obtained indicate that this type of bath does not cause a degradation of the fatigue life, which is the main drawback of the traditional sulfuric baths. The oxide film formed from the boric sulfuric electrolyte has a paint adhesion that is equal, or superior, to the one formed on chromic acid (Juhl, 2010).

9.8.1 Film structure Figure 9.7 shows TEM micrographs of aluminum (99%) anodized in a sulfuric bath for 30 min at 22 °C with a current density of 1.8 A.dm−2. The images depict both a barrier layer and a porous layer and show the cylindrical pores perpendicular to the surface. If aluminum is anodized in chromic acid at 40 °C applying a potential ramp from 0–22 V for 5 min and keeping this potential for 1 hour, the results reveal almost identical patterns, although the thickness of the barrier layer and pore dimensions are greater. When oxide films are formed in an AA2024 alloy, the porous layer structure changes to a granular pattern (non-oriented grain-like structure) separated by pores which are not perpendicular to the metal surface (Fig. 9.8). The different pore structure observed could be due to the presence of intermetallics where oxygen is likely to originate. Anodization using the sulfuric-boric process for 30 min at 22 °C with a current density of 1.5 A. dm−2 gives oxide film structures similar to those obtained with CAA and SAA electrolytes and again shows different patterns for pure aluminum and AA2024-T3. The same happens when the process is carried out at 40 °C, the main difference being the film thickness

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200 nm

Metal

100 nm

9.7 TEM micrographs of aluminum (99 %) anodized in sulfuric bath for 30 min at 22 °C. (Domingues et al., 2003b)

which changes from 11 to 2.5 μm. These results imply that the type of structure is characteristic of the alloy and of the electrolyte used for anodization (Domingues et al., 2003a).

9.8.2 Electrochemical characterization The electrochemical behavior of the anodized specimens was studied by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution. The

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9.8 Micrograph of transverse section of anodised AA2024 in BSA at 22 °C. (Domingues et al., 2003a)

impedance spectra shown as Bode diagrams for pure aluminum anodized in SAA and SBA baths and sealed in hot water are shown in Fig. 9.9. Two time constants can be seen corresponding to each of the oxide layers formed, i.e., the barrier and the porous layer. In the case of films formed in AA2024, the Bode plots reveal three time constants. In this case, due to the pore shape, the sealing is not complete, meaning that there is a hydrated sealed layer at the surface of the oxide but that in deeper regions of the film pores still exist. After a long time in an aggressive solution the pores can be penetrated and two time constants become apparent again (Domingues et al., 2003b).

9.8.3 Fatigue properties Figure 9.10 depicts the S–N plots of tensile–tensile fatigue tests leading up to the fracture of the specimens obtained for AA2024-T3 anodized with CAA, SBA and without treatment. An analysis of the results reveals that anodizing induces a decrease in fatigue resistance, which is more pronounced for the SBA process. However, this decrease in fatigue resistance is very low. The decrease can be characterized as (Sref – S) / Sref × 100, where Sref is the value of stress to fracture at 105 cycles for the reference treatment and S is the same value for the new surface condition. In particular, for SBA relative to CAA this ratio is ~2%, which is much lower than the scattering range of the results (Domingues et al., 2003a).

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8 7 6 log IZI

5 4 3 2 1

–θ

0 90 80 70 60 50 40 30 20 10 0 –2

–1 hours –3 hours –1 day –1

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6

log ω

9.9 Bode diagrams for commercial aluminum (99 %) coupons anodized in BSA solution for 30 min and sealed; different immersion times in 3 % NaCl solution. (Domingues et al., 2003a)

450 As-received

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400

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350 300 250 200 150 10 000

100 000

1 000 000

10 000 000

Cycles to failure

9.10 S–N plots obtained for AA2024-T3 in three different surface conditions: as-received, chromic acid anodized, ABS anodized. (Domingues et al., 2003a)

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9.9

Functional nanocoatings in aerospace engineering

The variety of conditions which can trigger corrosion processes poses serious challenges to engineers and scientists when designing functional coatings for the protection of metals and corresponding alloys. One of the most elegant strategies currently under development mimics the ability of living organisms to detect and heal bodily injuries (Ghosh, 2009). Specifically, there is a class of so-called ‘smart’ coatings whose novel functionalities are based on the release-on-demand of active species from micro- and nanostructured hosting structures embedded in the coating matrix. Self-healing by release of a healing agent upon mechanical damage was proved successful by the pioneering work of White et al. (2001). In this study, monomer-containing microcapsules are embedded into an epoxy matrix where the catalyst is dispersed (Fig. 9.11). Upon crack formation, microcapsules, located at and nearby the crack, break, releasing the mono-

Catalyst Microcapsule Crack

Healing agent

Polymerized healing agent

9.11 The self-healing effect by White et al. (2001).

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mers which, in contact with the catalysts, polymerize and seal the crack. Others have also used hollow fibers as reservoirs to increase the loading of healing agent (Trask and Bond, 2006). The main aim of corrosion prevention is to protect the metallic substrates and their function within a certain structure. The repairing effect must be driven to preserve the metallic substrate, in spite of the coating integrity being part of the protective solution. When the coating is damaged, aggressive species can reach the metal/polymer interface before it is repaired, causing underfilm corrosion even after the cracks or fractures have been sealed or healed. As a result, it is of paramount importance to include active functionalities in the coating formulation which contribute to the protection of the metal substrate in aggressive conditions, by detecting and mitigating corrosion activity. The former includes the monitoring of the rate of degradation and optimization of the timing of maintenance operations, while the latter involves the extension of coating protective action over time. In the next sections, new trends on nanostructured coatings for corrosion sensing and protection based on the controlled release of active substances from micro- and nanocontainers will be addressed. Finally, a short overview related to general aspects of protection using nanomaterials will be presented.

9.10

Nanocoatings for detection of corrosion and mechanical damage

The main function of (organic) coatings applied onto metallic substrates is to act as barrier against aggressive species from the surrounding environment, to avoid corrosion initiation and propagation, which can ultimately lead to structural failure. Due to aging effects or external factors including UV radiation, temperature gradients, mechanical stresses, or aggressive species (chlorides, H2O, pH), the occurrence of cracks and opening of pores is inevitable, leading to the ingress of corrosion-relevant species. To prevent the degradation of metals over time, the protective coatings can be provided with functionalities capable of sensing coating degradation, corrosion activity, or both. There are several works reporting the development of spectroscopic sensors (based on color changes, fluorescence, and phosphorescence detection) that respond to a variety of stimuli, including pH (Fig. 9.12), electric current, mechanical action, redox reactions, radiation, temperature changes, presence of metals, and sorption of chemicals (Feng et al., 2007). The encapsulation of ‘indicators’ in micro- and nanocapsules offers several advantages with respect to the direct dispersion of species in the formulation. Encapsulation limits the contact between indicating species and coating constituents, something which is particularly important if they cause changes in the surface chemistry of the formulation, and reduces the

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(a)

(b)

9.12 Color changes of two pH sensing coatings on aluminum alloys 5454 after immersion in 1.0 M NaCl solution: (a) phenolphthalein paint after 8 days; (b) bromothymol paint after 13 days. (Zhang and Frankel, 1999)

risk of loss of barrier properties. The dispersion of sensing species freely in the formulation can also lead to an erroneous detection of signal by interaction of the indicator with coating impurities or other kinds of interferences arising from earlier preparation steps (e.g. color changes due to pH treatments of the substrate). The capsules properties, such as shell permeability and mechanical stability, may also yield information on the order of magnitude of stimuli which causes the release of the indicator. In addition, the encapsulation of species provides localized sensing functionality because only the reservoirs in close vicinity to where degradation processes take place will respond to the stimulus, thereby preventing misleading results associated with the diffusion of the indicator dispersed in the coating matrix and/or heterogeneous distribution of these species. These points are particularly important in the aeronautical industry as very often the magnitude of mechanical damage inflicted in the metallic structure is vital to assess the level of maintenance operation required. Although methods of corrosion detection using color-stimulated species dispersed in the coating matrices are well known and subject to serious development effort, methods using micro-encapsulated chemical indicators are in their infancy. There are not many reports available in the literature concerning the encapsulation of indicators. In the work carried out by Li and Calle (2007), Li et al. (2009), and Calle and Li (2010) microcapsules loaded with color or fluorescent dyes and corrosion inhibitors were prepared, and corrosion tests were performed in steel and aluminum panels coated with paints displaying these dual functions. The results showed improved corrosion protection and also allowed visual detection of corrosion sites. Similar work was carried out by Kumar and Stephenson using

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micro-encapsulated fluorescent additives (Stephenson, 2008). Other strategies include the use of mesostructured silicas (Miled et al., 2004) and exfoliated ion-exchange nanoclays (montmorillonite and hydrotalcite) as hosts for pH color-responsive species (Xanthos et al., 2003a,b; Gopakumar et al., 2005; Xanthos et al., 2005, 2006; Zunino et al., 2005).

9.11

Self-healing coatings: nanostructured coatings with triggered responses for corrosion protection

The protective coatings currently used in the aerospace industry were discussed in previous sections. Traditionally, the most common approach for the active protection of metal alloys is based on organic coatings doped with corrosion inhibitors, most notably hexavalent chromium-derived species. Due to high toxicity, however, chromates are about to be prohibited, leaving a technological gap since the current available corrosion inhibitors are not capable of providing comparable anti-corrosion protection. Furthermore, the direct addition of inhibitors to the coating formulations causes several problems. The spontaneous leaching of corrosion inhibitors to the environment, regardless of the occurrence of corrosion, is both environmentally and economically undesirable limiting the protection to a relatively short period. The presence of soluble corrosion inhibitors can also cause osmotic blistering. In addition, corrosion inhibitors may react with the coating matrix leading to inhibitor deactivation, coating degradation, or both. One way of overcoming these issues is to encapsulate or intercalate the corrosion inhibitors in nanostructured, inert host materials, referred to hereafter as micro/nanocontainers, whose main function is to entrap the inhibitors for indeterminate periods of time until certain conditions, corresponding to coating degradation and corrosion initiation, are verified. There are currently several groups focused on the development of ‘smart’ nanocontainers for the controlled release of corrosion inhibitors. In principle, the utilization of nanoscaled containers is important for aeronautical applications as they can be added to different coating layers such as thin pre-treatments or thicker primer and topcoat layers, without limiting the coating integrity. Khramov and colleagues reported the immobilization of organic corrosion inhibitors by using cavities in cyclodextrins as recipients for the inhibitors (Khramov et al., 2005) and consequently incorporated them into sol–gel formulations. The results showed that an AA2024 substrate coated with these functionalized coatings displayed better anti-corrosion performances compared to undoped ones and to coatings with inhibitors directly dispersed in the matrix (in this case, the release of the inhibitors is not triggered by any corrosion-induced conditions). Sol–gel formulations with

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– film with Ce-doped ZrO2 – film with Ce-doped matrix –80 – undoped film

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active protection functionalities were obtained using ZrO2 nanoparticles as matrix reinforcements and adsorption sites for corrosion inhibitor Ce(NO3)3 (Zheludkevich et al., 2005a,b). After long immersion times in a NaCl solution, the anti-corrosion performance of coatings on AA2024 was found to be the highest in the ZrO2-Ce-containing sol–gel and the lowest in the undoped matrix (Fig. 9.13). The adsorption of cerium cations to the oxide nanoparticles also improves the barrier properties and slows down the release of inhibitor to solution, in comparison to the Ce-doped matrix. More complex and corrosion-relevant triggered release mechanisms can be achieved through the utilization of ion-exchangers as hosting structures for the corrosion inhibitors. The mechanism associated with ion-exchangers is simple, but effective: ionic inhibitors intercalated in such structures can be exchanged with ions available in the surroundings, such as when coating degradation occurs and allows the ingress of electrolyte species. An example is cation-exchanger bentonite which consists of stacks of negatively charged alumosilicate sheets, between which inhibiting cations can be intercalated (Bohm et al., 2001; Buchheit et al., 2002; McMurry et al., 2003). Ca2+- and Ce3+-loaded bentonites dispersed in polyester primer layers and applied to hot dip galvanized steel substrates display active protection, avoiding coating delamination. In spite of the positive effect on corrosion protection, the release of the inhibitor in this case is triggered by metal cations available in the surroundings. These may not be directly associated with corrosion, thereby leading to the waste of inhibitor. In the case of anion-exchangers, such as hydrotalcite-like materials, also called layered double hydroxides (LDHs), the structure consists of layers of positively-charged, mixed-metal hydroxides separated by layers of anions and water molecules. As a result,

0

Frequency (Hz)

9.13 Electrochemical impedance spectra of AA2024 substrates coated with different hybrid films after 250 hours immersion in NaCl solution. (Zheludkevich et al., 2005b)

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the triggering is more corrosion-relevant than in cation-exchangers, because the most abundant anions in nature (chlorides, sulphates) work as corrosion-inducing agents. Thus, the LDHs play a dual role (Fig. 9.14): the release of the corrosion inhibitor and the entrapment of aggressive species. Even the single trapping function of LDHs provides a positive effect as highlighted in the work of McMurray and Williams (2004). Buchheit et al. (2003) have shown the corrosion protection conferred by Zn–Al–decavanadate LDHs when added to epoxy coatings and applied to AA2024. The protecting action is due to the release of decavanadate and the uptake of chloride ions. Williams and McMurray (2004) also showed the versatility of the LDH structure by intercalating different organic corrosion inhibitors and assessing the filiform corrosion on coated AA2024. The performance of LDH-loaded nanocontainers did not, however, outperform chromate-based pigments. Ferreira and colleagues have also focused on the development of coatings loaded with LDH smart nanocontainers (Poznyak et al., 2009; Tedim et al.,

LDHs intercalated corrosion inhibitors (Inh–)-

Cl–

Cl–

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–

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–

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Cl–

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9.14 LDHs dual role in corrosion protection: (I) release of inhibitors and (II) entrapment of aggressive species. (Tedim et al., 2010)

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2010; Zheludkevich et al., 2010). Different corrosion inhibitors, including vanadates, phosphates, mercaptobenzothiazolate, and quinaldate, were intercalated in the LDH structure. The obtained LDH polycrystallites were found to be nanosized and plate-like structures. Release studies performed in solution demonstrated that the exchange of anions is fast and governed by equilibrium constant (Poznyak et al., 2009; Zheludkevich et al., 2010). Furthermore, vanadate-containing LDHs were added to an epoxy primer used in the aeronautical industry, and both electrochemical and standard accelerated tests showed a good active protection. In some of the accelerated tests, LDH-vanadate outperformed chromate-doped primer (Zheludkevich et al., 2010). Another important aspect considered was the enhancement of anti-corrosion performance by combining different nanocontainer/inhibitor pigments in different coating layers (multi-level protection) (Tedim et al., 2010). This is an alternative approach to solving technical problems including the low loading of corrosion inhibitors or the negative effect of using a high concentration of nanocontainers in the coating barrier properties. The coating used for this study consisted of a sol–gel pre-treatment, a water-based epoxy primer, and an epoxy-based topcoat. Electrochemical studies showed that LDH nanocontainers added to both the pre-treatment and the primer contribute to the improvement of both native aluminum oxide film and coating stabilities. An alternative approach for the controlled release of corrosion inhibitors consists of encapsulation using layer-by-layer (LBL) assembled shells. LBL deposition of oppositely charged monolayers on a nanostructured template allows control of the release of ions or small organic molecules, by tuning shell permeability. The storage of corrosion inhibitors in between polyelectrolyte layers isolates the inhibitor from the coating matrix and imparts an intelligent release dependent on pH and humidity. In the works of Zheludkevich and co-workers, silica nanoparticles were used as templates for benzotriazole encapsulation using polyelectrolyte shells (Shchukin et al., 2006; Zheludkevich et al., 2007). The resulting nanocontainers were added to a hybrid sol–gel formulation and applied in AA2024 plates. The samples coated with LBL nanocontainer-doped sol–gel films exhibited enhanced corrosion protection in comparison to the undoped and benzotriazole-directly-dispersed formulations. Using electrochemical localized techniques, the corrosion activity in artificially induced defects was not observed for the first few hours of immersion in the case of LBL nanocontainer-doped film, contrasting with the undoped coating that showed a growing cathodic activity with immersion time (Zheludkevich et al., 2007). Interestingly, the nanocontainer-doped sol–gel film showed cathodic activity after 24 hours of immersion, but the defect became passivated 2 hours later (Fig. 9.15), which can be interpreted as a self-healing effect caused by the release of inhibitor due to local changes in the pH, increasing the shell permeability and enabling the release of the inhibitor. Other templating © Woodhead Publishing Limited, 2011

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(a) (b)

Initial surface

Formed defect

(f) Formed defect

After 4 h of immersion

(g)

(c)

(d)

(e)

After 24 h of immersion 10 8 6 4 2 0 –2 –4 –6 –8 –10 After 48 h of immersion

(h)

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9.15 Scanning vibrating electrochemical technique (SVET) maps of ionic currents measured above the surface of AA2024, coated with undoped sol–gel pre-treatment (a, c, d, e) and with pre-treatments impregnated with nanocontainers (g, h, i). The maps were obtained before defect formation (a) and 4 hours (c, g), 24 hours (d, h) and 48 hours (e, i) after defect formation. Scale units: μAcm−2. Scanned area: 2 mm × 2 mm. (b, f) Optical micrographs of AA2024 Coated with sol– gel, displaying artificial defects (200 µm in diameter) formed on the surface of both coatings after 24 h of immersion in 0.05 M NaC1 solution. (Zheludkevich et al., 2007)

structures with higher loading capacities were also tested. Halloysite nanotubes comprise a two-layered aluminosilicate with a hollow tubular structure of submicron dimensions. In the work by Shchukin et al. (2008) halloysite nanotubes were loaded with 2-mercaptobenzothiazole (organic corrosion inhibitor) and subsequently coated with polyelectrolyte multilayers to prevent the spontaneous leaching of the inhibitor. The resulting nanocontainers were added to a hybrid sol–gel formulation consequently used to coat the AA2024 plates. The corrosion tests demonstrated superior protective action of the doped coating compared to the undoped one. © Woodhead Publishing Limited, 2011

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The rupture of micro- and nanocapsules upon mechanical damage is also a relevant mechanism for the corrosion protection of metallic substrates used in the aeronautical industry. Macroscopically-induced defects such as scratches are a source of regular damage to protective coatings. Kumar and colleagues presented a detailed study on the feasibility of developing selfhealing polymers using commercially available formulations and embedding microcapsules loaded with corrosion inhibitors and other healing agents (Kumar et al., 2006). The results showed the release of the encapsulated active species under abrasion and the consequent reduction of undercutting at scribes. Raps and colleagues recently reported the synthesis of polymeric microcapsules prepared by microemulsion polymerization, loaded with organic corrosion inhibitor 2-mercaptobenzothiazole (2-MBT) and 8-hydroxiquinoline (8-HQ) (Raps et al., 2010). The capsules were then added to a water-based epoxy primer applied on top of pre-treated AA2024 substrates with anodizing or sol–gel films, and finally overcoated with a water-based epoxy topcoat. Good dispersion and adhesion between capsules and the coating matrix was achieved. Additionally, the electrochemical measurements showed that paints impregnated with 2-MBT-loaded microcapsules exhibited better barrier properties than chromate-loaded primers and that the self-healing of artificially induced defects was more prominent in the former case. Filiform corrosion tests also pointed to the better performance of coatings with 2-MBT-loaded microcapsules, although coating blistering was observed for high loadings of corrosion inhibitor. There are also several patents based on coatings doped with corrosion inhibitor-loaded capsules to be triggered by mechanical damages, which demonstrates the importance of this type of functional coatings (Sayre and White, 2007; Schroeder et al., 2008), particularly for the aeronautical industry (Gammel et al., 2009).

9.12

Application of nanomaterials for protection of aeronautical structures

The control of coating structures at nanometer level is very important for obtaining specific functionalities and maximizing their performance. In previous subsections the incorporation of ‘smart’ micro and nanoreservoirs was shown to improve corrosion detection and protection. Herein, new trends related to other forms of protection, equally important to increasing coating service life and reducing maintenance costs, will be highlighted. Ice accumulation is an important issue not only for aircraft, but also for other systems like wind power structures (Dalili et al., 2009). Ice accretion causes a disruption of the aerodynamics by increasing surface roughness and drag coefficients, and decreases fatigue life (Jasinski et al., 1998; Antikainen and Peuranen, 2000; Talhaug et al., 2005). The performance of

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turbines in cold weather, such as the negative temperatures experienced at high altitudes, may also have an impact on the physical properties of the coating and the underlying metallic structures. Another problem associated with turbine function is erosion provoked by the impact of solid particles and water droplets, which contributes to a decrease in aerodynamic performance and reduction in working power. One of the most common approaches to reducing ice accretion in windmill turbines is heating, using electrical resistances, microwave, or indirect heating (warm air inside the blades or using a radiator). There are, however, several negative issues associated with this method, including the attraction of lightning by the electrode elements such as metal and carbon fibers, the increase in the surface roughness of the blades due to the presence of embedding electrodes or fibers, and electricity consumption, etc. Current research is focusing on the development of nanocomposite coatings where the nanometer-sized particles act as a reinforcement of the polymeric structure. Nanocomposites show very high contact angles with water and, at the same time, the reinforced polymer absorbs and dissipates high energy impacts. Some examples include the use of electrically conducting particles to increase the temperature, such as nanotubes, metal nanorods, functionalized metal nanoparticles, nanostructures of carbon nanotubes or fullerenes grafted to a polymer containing an active functional group (Heintz et al., 2008), and inorganic–organic coatings, with the inorganic component imparting hardness and durability and the organic counterpart flexibility and icephobicity (Taylor, 2004). Berman and colleagues recently presented a work on the development of clear-coat based on phase changed materials (PCM) with anti- and de-icing properties (Berman et al., 2009) (see Fig. 9.16). The PCMs undergo solid-phase changes causing small localized structural changes in the coating, creating stress and strain at the ice–coating interface. As a consequence, a dramatic reduction in the force necessary for removal of accumulated ice is required. Most of the information in this field, however, is still proprietary, with several patents claiming novel anti-icing and erosion-resistant coatings. A patent by Blackwell reports a new anti-icing/de-icing formulation comprising a non-toxic and freezing point depressant, optional non-toxic thickener, and a polymer. The formulation forms moisture-sensitive nanoparticles or nanospheres encapsulating the freezing point depressant or other type of species such as oleophobic or traction increasing agents (Blackwell, 2010). The resulting composition is claimed to repel water, prevent ice formation, and avoid corrosion. A patent by Doll et al. (2004) reports the synthesis of a functional coating consisting of a gradient layer and a functional, thin (0.1–1 μm) top layer comprising hydrogen, silicon, and oxygen. The overall coating is hard, wear resistant and ice-phobic and can be applied in wings, propellers, or aircraft fuselages.

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PCM structures

Resin matrix Moisture

9.16 Deicing Process of ePaint’s PCM-based ice-phobic coatings. (Berman et al., 2009)

Mons (2009) invented an anti-icing coating formulation based on a phenolic resin in which defrosting and/or anti-icing containing polymeric capsules are embedded. The defrosting/anti-icing species are released upon rupture. Another patent by Jing et al. (2007) reports the synthesis of a superhydrophobic surface using a formulation comprising binder and hydrophobic micro- and nanoparticles on a micropatterned surface which can potentially be used in anti-icing applications. The self-cleaning of coating surfaces is also an important aspect for maximizing the service life of coated substrates. The consequences associated with the impact and accumulation of dirt and/or living organisms are similar to icing accretion in terms of coating degradation and the measures used to prevent it. Most of the state of the art technology is proprietary and only some patents are known. The suggestions in these patents include hydrophobic surfaces obtained by the deposition of nanowires on the substrates

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(Choi et al., 2010), coatings with photochemically-active semiconductor oxide nanoparticles which destroy biological and chemical agents (Chu et al., 2010), a superhydrophobic coating based on nanocomposites of carbon nanotubes and oligo(p-phenylenevinylene) (Ajayaghosh et al., 2009), and nanofibers combined with liquidphobic material (Dubrow, 2005). The protection of turbine blades from high temperature oxidation, corrosion, and erosion cannot be achieved using organic-based coatings. Instead, functional gradient ceramic/metallic coatings produced by high energy methods have been applied (Wolfe and Singh, 1998). There are several available reviews reporting advances in coating design and production for turbine-related applications (Nicholls, 2003; Montavon, 2004; Singh and Wolfe, 2005; Fauchais et al., 2008). The following paragraphs discuss the manipulation of the structure of some of these coatings at nanoscale. Rao and colleagues studied the synthesis of carbon/titanium diboride multilayer coatings by DC magnetron sputtering on steel substrates (Rao et al., 2004). Titanium diboride is known for exhibiting great hardness and a high melting point and level of corrosion resistance, which make its potential application in the aeronautical industry very promising. Its application has been limited, however, by its brittleness. The reported multilayer design with carbon (alternating films of TiS2 and carbon with thicknesses of 100 nm or less, making a total of 5 μm) prevents crack propagation due to the existence of a large number of interfaces. Yoon and colleagues reported the synthesis of a nanostructured TaSi2 coating reinforced with Si3N4 nanoparticles produced on a tantalum substrate (Yoon et al., 2008) (Fig. 9.17). The results showed no cracks, and the isothermal and cyclic oxidation of the nanocomposite coating between 1300 and 1400 °C was superior to that of the TaSi2 coating without the nanoparticles. Gorokhovsky et al. (2007) produced hybrid films of multilayered nanocomposite coatings TiCrN/TiCrCN+TiBC using a hybrid-filtered

(a)

(b)

Si3N4

Si3N4

TaSi2

100 nm

TaSi2

100 nm

9.17 XTEM bright field images of (a) outer layer and (b) inner layer of the TaSi2–Si3N4 nanocomposite coating. (Yoon et al., 2008)

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arc-magnetron deposition method on steel substrates. The control of the coating structure and architectures at nanometer level has been proven to be a successful way to improve corrosion resistance and tribological performances. Other thermal barrier coatings with modified nanostructures include modified zirconia oxide (Saruhan et al., 2006; Olszyna and Kostecki, 2010), which is the current state of the art thermal barrier coating for use in aircraft engines.

9.13

Conclusion and future trends

The different types of coatings and corresponding functionalities presented underline the necessity for a careful understanding of the specific protective characteristics of aeronautical structures. There is no single solution to protect the entire aircraft structure. Instead, an integrated approach must be followed, combining coating layers and functionalities in order to achieve higher protective performances and, consequently, a longer service life of the structures, as well as reducing the costs associated with maintenance operations and increasing the energy efficiency of aircraft during operation. Based on the available preliminary information, the development of sensorbased, corrosion active and anti-icing/self-cleaning smart coatings is perhaps the best step towards prospective future functional materials. Both fundamental and applied research in this area is expected to grow in the near future, contributing to a new generation of high performance, added-value products in the forthcoming years.

9.14

References

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Williams G and McMurray HN (2004), ‘Inhibition of filiform corrosion on polymer coated AA2024-T3 by hydrotalcite-like Pigments Incorporating organic anions’, Electrochemi Solid State Lett, 7, B13–B15. Wolfe D and Singh J (1998), ‘Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications’, J Mate Sci, 33, 3677–3692. Xanthos M (2005), ‘Smart materials in polymer protective coatings and films – corrosion sensors, barrier properties, self-healing’ Proceedings of Pira International Conference on ‘The Future of Nanomaterials’, 22–24 February, Miami, FL. Xanthos M (2006), ‘Functional nanoclays as corrosion sensors in smart polymer coatings’ Proceedings of US Army Corrosion Summit 2006, 14–16 February, Clearwater Beach, FL, Session B-Day 2, p. 11. Xanthos M, Chouzouri G, Kim S, Patel SH and Young M-W (2003a), ‘Functional Additives as sensors in intelligent polymer coatings’, Proceedings of European Coatings Conference, Smart Coatings II, 16–17 June, Berlin, Germany. Xanthos M, Chouzouri G, Kim S, Patel SH and Young M-W (2003b), ‘Zur sache: additive – Reiz und reaktion – funktionelle additive als sensoren in intelligenten Lacken’, Farbe & Lack, 109(8), 18–23. Yang XF, Tallman DE, Gelling VJ, Bierwagen GP, Kasten LS and Berg J (2001), ‘Use of a sol–gel conversion coating for aluminum corrosion protection’, Surf Coat Technol, 140, 44–50. Yasakau KA (2011), Active Corrosion Protection of AA2024 by Sol–Gel Coatings with Corrosion Inhibitors, Doctor thesis, Universidade de Aveiro, Portugal. Yasakau KA, Zheludkevich ML, Lamaka SV and Ferreira MGS (2006), ‘Mechanism of corrosion inhibition of AA2024 by rare-earth compounds’, J Phys Chem B, 110, 5515–5528. Yoon J-K, Kim G-H, Kim H-S, Sho I-J, Kim J-S and Doh J-M (2008), ‘Microstructure and oxidation behavior of in situ formed TaSi2–Si3N4 nanocomposite coating grown on Ta substrate’, Intermetallics, 16, 1263–1272. Yu X and Cao C (2003), ‘Electrochemical study of the corrosion behavior of Ce sealing of anodized 2024 aluminum alloy’, Thin Solid Films, 423, 252– 256. Zhang J and Frankel GS (1999), ‘Corrosion-sensing behavior of an acrylic-based coating system’, Corrosion, 55, 957–967. Zheludkevich ML, Serra R, Montemor MF, Yasakau KA, Salvado IMM and Ferreira MGS (2005a), ‘Nanostructured sol–gel coatings doped with cerium nitrate as pretreatments for AA2024-T3 – Corrosion protection performance’, Electrochim Acta, 51, 208–217. Zheludkevich ML, Serra R Montemor MF and Ferreira MGS (2005b), ‘Oxide nanoparticle reservoirs for storage and prolonged release of the corrosion inhibitors’, Electrochem Commun, 7, 836–840. Zheludkevich ML, Yasakau KA, Poznyak SK and Ferreira MGS (2005c), ‘Triazole and thiazole derivatives as corrosion inhibitors for AA 2024 aluminium alloy’, Corros Sci, 47, 3368–3383. Zheludkevich ML, Serra R, Montemor MF, Salvado IMM and Ferreira MGS (2006), ‘Corrosion protective properties of nanostructured sol–gel hybrid coatings to AA2024-T3’, Surf Coat Technol, 200, 3084–3094.

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10 Nanoimprint lithography (NIL) and related techniques for electronics applications I. TIGINYANU, V. URSAKI and V. POPA, Academy of Sciences of Moldova, Republic of Moldova

Abstract: This chapter provides a review of nanoimprint lithography techniques, highlighting their potential to surpass photolithography in resolution, and, at the same time, to allow mass fabrication at a lower cost. The current and potential uses of nanoimprint lithography are discussed in fields such as data storage, optical components, image sensors, and phase change random access memory devices. Challenges faced by nanoimprint lithography in becoming a standard fabrication technique are considered in connection with recent technology developments to extend existing optical lithography processes for semiconductor fabrication. Key words: nanofabrication, nanoimprint, patterning, thermal imprint, soft lithography, mold, polymer resist, thermoplastic.

10.1

Lithography techniques and nanoimprint lithography (NIL) fundamentals

Nanofabrication represents the most important basis for the evolution of electronics, optoelectronics, photonics, and information technologies. One of the key processes in the fabrication of functional devices is patterning. Patterning is commonly referred to as lithography and it has, in the past, been accomplished almost entirely by photolithography. However, nanofabrication requires the introduction of unconventional techniques for material patterning to make functional structures. Usually, a lithographic technique includes the following elements (Geissler and Xia, 2004): • • • •

a pre-designed set of patterns in the form of mask or master; utilities to mediate the transfer or replication of patterns; a functional material capable of serving as the resist for subsequent steps; metrological tools.

Lithography techniques can be classified as either conventional or unconventional as shown in Table 10.1. Conventional techniques are divided into 280 © Woodhead Publishing Limited, 2011

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1. With a beam of photons 2. With electrons 3. With ions 4. With neutral metastable atoms

1. Photolithography (PL) 2. UV-lithography 3. X-raylithography 4. Interferometric lithography UV-NIL Step and flash imprint

Nanoimprint

Scanning beam writing

Projection lithography

Thermal-NIL 1. With hot embossing 2. With thermopolymerization 3. Laser-assisted

Unconventional lithography

Conventional lithography

Table 10.1 The nomenclature of lithography methods

Soft-NIL 1. Micro-transfer printing (μTP) 2. Nano-transfer printing (nTP) 3. Solventassisted micromolding (SAMIM) 4. Micromolding in capillaries (MIMIC) 5. Capillary force lithography (CFL) 1. Electrical microcontact printing 2. Electrochemical lithography 3. Surface charge lithography 4. Photocatalytic lithography

Extension of nanoimprint

1. With a rigid stylus 2. With an add-on process 3. With selective removal 4. With magnetic field 5. With electric field

Scanning probe

1. Near-field phase-shifting PL 2. Topographically directed PL 3. Controlled undercutting 4. Topographicaly directed etching 5. Step-edge decoration

Edge lithography

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projection lithography (making use of masks) and scanning beam (maskless) lithography. These techniques are highly developed and widely used for fabricating microelectronic circuits. The most common type of projection lithography is photolithography. Photolithography is a patterning process in which a photosensitive polymer is selectively exposed to light through a mask, leaving a latent image in the polymer that can then be selectively dissolved to provide patterned access to an underlying substrate. The photoresist is usually a photosensitive polymer that undergoes a chemical reaction upon exposure to light. For a large-scale application, any lithographic technique should meet stringent requirements in terms of throughput, overlay accuracy, and resolution. The critical feature size of the patterns is one of the most important issues to be considered when selecting a proper lithographic technique. Diffraction of the light used to expose a photoactivated film of resist limits the dimensions of the features produced by photolithography. Traditionally, the semiconductor industry has achieved continuing reductions in device size with optical photolithography by decreasing the exposure wavelength with accompanying improvements in the photoresist chemistry and optics, and a variety of resolution enhancement techniques (Lin, 2006; Pease and Chou, 2007). Photolithography can pattern 37 nm wide features with 193 nm wavelength light. Patterning features below 37 nm using photolithography is possible with optical proximity correction (OPC) or phase-shifting mask technology which significantly increases the cost of photomasks, or with ‘immersion lithography’. Immersion lithography improves resolution by increasing the refractive index of the medium between the imaging lens and the imaging plane. To pattern still smaller features, photolithography requires further decreasing the imaging wavelength. However, the costs for optical photolithography increase significantly on the wavelength decreases. The shift in wavelength below 157 nm (deep UV and soft x-ray wavelengths) brings increasing technical difficulty, particularly in developing new optical elements (Burkhardt et al., 1995; Levenson, 1995; Cerrina and Marrian, 1996). According to the International Technology Roadmap for Semiconductors (ITRS), published by the Semiconductor Industry Association (SIA), the 45 nm node began to be manufactured in 2010 and the 32 and 22 nm nodes are at the research and development stage. Extreme UV lithography (EUV), in which a 13.5 nm source wavelength is used for projection patterning with reflective optics, has the potential to reach the 32 nm or 22 nm node. However, it has a number of drawbacks (Costner et al., 2009). The expected cost of an EUV tool, $54–89 million, is significantly greater than costs for previous generations of tools and, at current throughput levels, is prohibitive. Some of the critical issues for EUV include a source of sufficient intensity to overcome reflection losses through the optical elements, a resist of high sensitivity that has acceptable line edge

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roughness, and a mask that can maintain low defect levels without the use of a pellicle. Scanning beam lithographic techniques, which include writing with a beam of photons, electrons, ions, or neutral metastable atoms, offer alternative approaches to patterning small features. However, these techniques are slower than projection lithography. A pattern is carved out line-by-line by scanning a beam of particles over a resist material. The physical and chemical processes used for writing in a film of resist are typically polymerization, depolymerization, or ablation. Scanning with laser beams with ∼250 nm resolution is the least expensive technique. The resolution of electron beam lithography (EBL) is limited to around 10 nm, while a focused ion beam (FIB) can deposit or remove material with resolution down to around 5 nm. However, these techniques, depending on tool settings and the choice of photoresist, are expensive to purchase and maintain. FIB systems with sub-50 nm resolution are primarily used in research. Typically, high resolution photomasks are patterned using laser writers and electron-beam tools. Photolithography has a number of advantages over scanning beam lithography in nanofabrication, but the time and cost required to fabricate the photomasks which are typically patterned by scanning beam lithography can be a significant drawback. However, for specific applications (e.g., diffraction gratings) one can apply interferometric lithography which is a cost-effective photolithographic method without using a photomask or most of the expensive projection optics (Zaidi and Brueck, 1993; Bozler et al., 1994). This process involves constructive and destructive interference of multiple laser beams at the surface of a photoresist. Each of these conventional fabrication tools, except for soft x-ray lithography, is commercially available, and they are highly developed and optimized for semiconductor fabrication. However, their high costs are limiting factors for application in areas other than microelectronics. For instance, it is difficult to pattern many organics and biological samples using photolithography. These circumstances stimulate efforts to develop alternative tools for nanofabrication. A number of unconventional methods are under development to circumvent limitations and to enable new types of fabrication. One can classify unconventional methods into several categories, such as nanoimprint, extension of nanoimprint, scanning probe, edge lithography. Here, we will describe nanoimprint lithography (NIL) in more detail. According to the nomenclature presented in Table 10.1, NIL is classified into three groups: thermal NIL, UV-NIL (or step and flash imprint), and soft NIL, which includes nanotransfer printing (nTP), microtransfer printing (μTP), and capillary force lithography (CFL). However, this nomenclature is flexible, and the same method can be attributed to several groups. For instance, a portion of UV-NIL methods can be

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attributed to soft lithography methods. Moreover, according to the classification of Gates et al. (2004), all the nanoimprint methods are attributed to the area of soft lithography. Different schemes of classification arise from different concepts. If one considers soft lithography according to applied pressure, i.e. if one attributes to soft lithography all the methods with a low applied pressure, then a considerable portion of thermal NIL and UV-NIL will fall into the category of soft methods. The classification in Table 10.1 is according to the principle of operation. The differences between various techniques are primarily in the type of material used for the template and for the imprintable resist material. Within each primary technique, there are numerous variations. Nanoimprint lithography is a process in which a mold (also called a template) is pressed into a deformable material to form a pattern. The material is then hardened so that when the mold is removed, the topography of the mold is transferred into the material. The low cost and simplicity of this technique, and its success in transferring nanoscale patterns with high fidelity, make it attractive for a wide range of applications which will be described in more detail in the next sections. The demonstrated resolution of imprint techniques has led to the inclusion of NIL on the ITRS, published by the SIA as a potential technology for patterning 32 nm or 22 nm features (Semiconductor Industry Association, 2007). Moreover, the resolution potential has been demonstrated by the replication of 2.4 nm features (Hua et al., 2004). Since NIL is a mechanical process, the resolution is not limited by the diffraction of light or the photoresist chemistry and development, as in optical photolithography. Instead, the resolution of NIL is determined by the minimum template feature size that can be fabricated. As the template fabrication process improves, imprint resolution also improves. The ability to imprint both functional materials and three-dimensional (3D) structures is an advantage of NIL. In conventional lithography, patterning is limited to two dimensions. Therefore, multiple lithography and etch steps are required to generate a 3D pattern. On the other hand, imprint can generate the same structure with one imprint step using only a simple equipment set-up, leading to high throughput and low cost processes. Because 3D templates can be fabricated, NIL can be used to pattern 3D structures. Further advantages can be explored by combining a 3D imprint with an imprintable functional material. The three NIL techniques mentioned in Table 10.1 are briefly described in Fig. 10.1. All the NIL techniques can be described in terms of four process steps. Thermal NIL was first demonstrated in 1995 (Chou, et al., 1995). The process used a silicon stamp to pattern a thermoplastic polymer at elevated pressure and temperature. The thermoplastic polymer is coated on a substrate in the first step (Fig. 10.1a). In this thermal imprint technique (also

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(a)

(b)

285

(c)

Template

Template

Template

Substrate stack

Substrate stack Apply pressure

Substrate stack

Step 2

Step 3

Step 4

10.1 Process steps for the three basic NIL techniques: (a) thermal NIL, (b) UV-NIL, (c) a soft lithography method (microcontact printing).

called hot embossing), the template is pressed into the thermoplastic film in the second step while the film is heated above its glass transition temperature. The heated viscous material flows and fills the mold with the application of high pressure. The pattern is then cooled, and the mold is removed in the third step to leave the pattern in the film. The fourth and final step is the breakthrough and pattern transfer etching. UV-NIL (also called step and flash imprint) is a room temperature and low pressure imprint technique in which the imprint material is a photocurable, low viscosity monomer solution and the template is transparent (for instance, quartz). After the imprint material (e.g., a monomer) is dispensed dropwise on the substrate in the first step, the template is brought into close proximity to the substrate so that the imprint material flows to fill the template pattern through capillary action, without the application of elevated pressure and temperature (Fig. 10.1b). The monomer is then exposed to UV light in the second step through the transparent template to photopolymerize the imprinted features. Thus, the material is changed from a liquid to a solid by a photochemical reaction. An inverse replica of the template pattern is formed in the photopolymer in the third step when the mold is removed. The fourth and final step is similar to that applied in the thermal NIL, and it comprises breakthrough and pattern transfer etching. The UV-NIL process was developed in 1999 (Colburn et al., 1999) and was effectively commercialized (Stewart et al., 2005; Resnick, 2007).

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Soft lithography was first developed in 1994 (Kumar, et al., 1994) and generally refers to imprint techniques in which a soft (elastomeric) polymer, such as poly(dimethylsiloxane) (PDMS), is used as template. A version of the soft lithography is illustrated in Fig. 10.1c, where the template is ‘inked’ with alkanethiol in the first step, and the ‘ink’ self-assembled monolayer (SAM) is transferred onto a substrate in the second step. The next two steps, i.e. the separation of the template and SAM (step 3) and breakthrough and pattern transfer etching (step 4), are similar to those applied in the thermal NIL and UV-NIL. Since the flexible elastomeric stamp can be deformed reversibly and repeatedly without permanent distortion, soft lithography can also be used to transfer patterns into non-planar surfaces. As mentioned above, a variety of methods using a soft elastomeric stamp are considered to be versions of soft lithography, including molding, printing, and embossing. For instance, μTM is widely used for the production of patterned microstructures of a wide variety of polymers, optical waveguides, couplers, and interferometers from organic polymers. In μTM, a thin layer of liquid pre-polymer is applied to the patterned surface of a PDMS mold and the excess liquid is removed by scraping with a flat PDMS block or by blowing off with a stream of nitrogen. The filled mold is then placed in contact with a substrate and irradiated or heated. After the liquid precursor has cured to a solid, the mold is peeled away carefully to leave a patterned microstructure on the surface of the substrate.

10.2

Thermoplastic and laser-assisted NIL

There are many variations in thermal lithographic techniques using different materials for the mold and the thermoplastic. The mold material is selected taking into consideration hardness, compatibility with traditional microfabrication processing or the intended applications, and the thermal expansion coefficient of the material. The thermal expansion coefficient is especially important in the NIL process, where a temperature of more than 100 °C is typically required at the imprinting step. A thermal mismatch between the mold and the substrate could result in pattern distortions or stress buildup during the cooling cycle, which would affect the pattern fidelity and registration accuracy. Silicon dioxide and silicon were used as the mold materials in the first works (Chou et al., 1995, 1996). The mold was patterned with dots and lines with a minimum lateral feature size of 25 nm using EBL and reactive ion etching (RIE). The beam lithography ensures that parameters such as the pitch and linewidth of the structures are very well defined and uniform across the entire stamp. The RIE ensures a very good height uniformity of the etched structures and also enables good control of the sidewall angle of the structures. In order to ensure the release of the stamp

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and substrate after imprint, the stamp is normally coated with an antisticking layer. Apart from RIE, nanostructured hard molds are prepared by transferring a structure patterned in photoresist into a hard substrate using wet chemical etching, or electroplating. In addition to the silicon dioxide and silicon used in the first works, hard molds have been fabricated out of quartz, ceramics, and metals (Gates et al., 2005). Candidate materials for molds include also silicon carbide, silicon nitride, sapphire, and diamond film (Guo, 2007). The smallest features transferred into a silicon mold are ∼10 nm wide lines written using electron-beam lithography (Chou and Krauss, 1997). The smallest features produced in a quartz mold are 20 nm (Resnick et al., 2003). Silicon and quartz molds are chemically inert to precursors used to mold polymers. Figure 10.2 shows some examples of molds with different periodical features fabricated by using a grating mold and performing NIL twice, at orientations of either 90° or 60° with respect to each other, followed by metal deposition, lift-off, and finally RIE to produce the desired mold features (Guo, 2007). These types of molds have been fabricated by Guo and co-workers and have been used to fabricate uniform and oriented metal nanoparticle arrays for studying localized surface plasmon resonances. Silicon gratings have been used as master molds for the fabrication of polymer imprint molds by hot embossing flexible fluoropolymer sheets of ethylene tetrafluoroethylene (ETFE) (Weiss et al., 2010). For high temperature imprinting, epoxy molds were also fabricated by standard softlithography methods using a silicon master mold and a PDMS silicone elastomer. The silicone elastomer was cast against a silicon master mold and

Acc.V Spot Magn WD 20.0 kV 3.0 75721x 7.7

(a)

500 nm

Acc.V Spot Magn WD 20.0 kV 3.0 75181x 8.9

(b)

500 nm

Acc.V Spot Magn Det WD Exp 20.0 kV 3.0 20634x SE 8.3 1

1 μm

(c)

10.2 SEM images of large-area molds with arrays of different features. (a) A pillar array, produced by imprinting twice with the same grating mold but orthogonal directions. (b) A bar array, produced by two grating molds with different periods; scale bar: 500 nm. (c) Diamond shaped array, produced by two imprints that use the same grating mold but are oriented at an angle of 60 °. (Reprinted from Guo LJ (2007), ‘Nanoimprint lithography: methods and material requirements’, Advanced Materials, 19 (4), 495–513. Copyright (2007) with permission from Wiley-VCH Verlag GmbH & Co. KGaA)

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was cured in an oven at 70 °C for 12 hours. After cooling to room temperature, the silicon mold was removed, and a drop of very low viscosity epoxy was placed on the PDMS mold. The epoxy was then cured in an oven at 70 °C for 2 hours and subsequently removed from the PDMS mold after cooling to room temperature which results in the production of an epoxy mold. In addition to ETFE and epoxy, two other mold materials, polyetherimide (PEI) and polycarbonate (PC), were tested. However, neither material produced a good imprint: demolding was difficult, and the molds were contaminated with resist after imprinting. These non-fluorinated thermoplastic polymers proved to be less hydrophobic than ETFE and epoxy. The fabrication of metal daughter molds has been demonstrated by electroforming in combination with hot embossing lithography (Heyderman et al., 2001). It has been shown that resolution down to 15 nm and high aspect ratios can be obtained using electroforming. Although silicon stamps have shown their capability for mass fabrication of nanostructures down to 25 nm, metal stamps still have an advantage due to their good mechanical properties, such as low notch sensitivity. Nanoimprint lithography using silicon nitride molds has been demonstrated at temperatures as low as 50 °C, well below the glass transition temperature of the poly(methyl methacrylate) (PMMA) (Alkaisi et al., 2001). No sticking problems were evidenced for this mold, so a surface release agent is not required. This is due to surface properties of the nitride where the adhesion of the PMMA to the nitride is weaker than to the Si substrate. It has been shown that high aspect ratio molds with deep intrusions are possible with the nitride. Since imprint lithography makes a conformal replica of the surface relief patterns by mechanical embossing, the resist materials used in imprinting need to be easily deformable under an applied pressure and should have sufficient mechanical strength as well as good mold-releasing properties to maintain their structural integrity during the demolding process. Good etching properties are also required for a subsequent RIE process. The resist material should have a Young’s modulus lower than that of the mold during imprinting, and the minimal pressure required to perform the imprint should be higher than the sheer modulus of the polymer. The resist material should also have a sufficiently low viscosity. For the plastic materials used in the thermal NIL process, the Young’s modulus and the viscosity can be decreased by several orders of magnitude compared to their respective values at room temperature by raising the temperature above the glass transition temperature. PMMA was a primary resist used in the first works (Chou et al., 1996), although success was reached with AZ and Shipley novlak resin-based resists as well. The PMMA showed excellent properties for imprint lithography. PMMA has a small thermal expansion coefficient of ∼5 × 10−5 per °C

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and a small pressure shrinkage coefficient of ∼3.8 × 10−7 per psi. Mold release agents were added into the resists to reduce the resist adhesion to the mold. The imprint temperature used with PMMA is typically between 140 and 180 °C, and the pressure is from 600–1900 psi. To reduce air bubbles, the imprint process should be done in vacuum. The gas used in the RIE pattern transfer, which also depends on the resist used, is oxygen for PMMA. Two of the most critical steps of NIL are mold release and pattern transfer through dry etching. These require that the NIL resist have low surface energy and excellent dry-etching resistance. Homopolymers traditionally used in NIL, such as polystyrene (PS) or PMMA, generally cannot satisfy all these requirements as they exhibit polymer fracture and delamination during mold release and have poor etch resistance. Recent progress in thermal NIL has focused on developing new resists, such as mixtures of PMMA and other materials, and on understanding the template filling process (Costner et al., 2009). A number of siloxane copolymers have been investigated for use as NIL resists, including poly(dimethylsiloxane)block-polystyrene (PDMS-b-PS), poly(dimethylsiloxane)-graft-poly(methyl acrylate)-co-poly(isobornyl acrylate) (PDMS-g PMA co-PIA), and PDMSg-PMMA (Choi et al., 2007). The presence of PDMS imparts the materials with many properties that are favorable for NIL, including low surface energy for easy mold release and high silicon content for chemical-etch resistance, in particular, extremely low etch rates (comparable to PDMS) in oxygen plasma, to which organic polymers are quite susceptible. These properties give improved NIL results. The development of new resist materials has led to increased throughput (Liao and Hsu, 2007). The effect of polymer molecular weight has been evaluated to understand the template filling process and to identify a critical molecular weight (Scheer et al., 2007; Bogdanski et al., 2008). The glass transition temperature for some polymers is given in Table 10.2. More sophisticated polymer systems include conducting polymers, polymers labeled with fluorescent chromophores, and block copolymers. Thermosetting polymers and solvent-containing hydrogen silsesquioxane or tetraethoxysilane compounds have also been used for thermal NIL (Nie and Kumacheva, 2008). The resists provided by commercial suppliers (NanoNex, microresist technologies GmbH or Sumitomo Ltd) have improved process properties such as enhanced etch resistance, lower glass transition temperature, lower viscosity, and enhanced mechanical strength (Schift, 2008). Weiss et al. (2010) described a nanoimprint method for an all-inorganic resist material, aluminum oxide phosphate. The resist is free of organic additives, water-based, environmentally benign and yields dense, amorphous, crack-, and pore-free films after annealing at 300 °C. Macroscopically defect-free imprinted areas of up to 25 cm2 were achieved, using flexible

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Nanocoatings and ultra-thin films Table 10.2 Glass transition temperature for some polymers Polymers

Trademark

Glass transition temperature, ˚C

Polymethyl methacrylate (PMMA) Polystyrene (PS) Polytetrafluoroethylene (PTFE) Polydimethylsiloxane (PDMS) PDMS–g–PMMA PDMS–g–PMA-co-PIA PS–b–PDMS

AR-P 669.04 95–105

95–105

168N (BASF) Teflon AF1601S

110 160 −127 105 54–64 −127, 105

ethylene tetrafluoroethylene imprint molds. It was shown that, if temperature and pressure are chosen such that the residual solvent in the resist stays liquid during imprinting, macroscopically defect-free imprints can be obtained. Zelsmann et al. (2008) compared a polymer resist (chemically amplified resist NEB 22, Sumitomo Chemical, Japan) to a thermally curable monomer resist (Laromer 8765, BASF, Germany) in a full 8 in. wafer thermal nanoimprint lithography process. It was shown that a liquid monomer solution greatly enhances the printing uniformity because of a much wider resist redistribution and flow during the process. Furthermore, a low molecular weight resist was found to allow reducing the imprinting force as well as the total cycle time. Another approach for imprint lithography consists in using a thermopolymerization reaction under the mold rather than the softening of a polymerized thermoplastic resist (Sagnesa et al., 2002). This approach has been demonstrated with methylmethacrylate (MMA) monomers. It was shown that the major problem with this technique is to keep a reasonable amount of the solution containing the monomer species in between the sample and the mold, due to its extreme fluidity and high evaporation rate. However, this problem was solved by using various silane molecular layers deposited on both the sample and the mold. Under appropriate conditions, no residual layer of polymer is left under the imprinted features, due to the concept of a minimum activation volume allowing the polymerization reaction to occur. This result opened the possibility to directly perform a transfer procedure like lift-off, without any need for a dry-etching process. Heating and melting of the thermoplastic resist can be performed also by laser radiation. A rapid technique for patterning nanostructures in silicon that does not require etching has been devised and demonstrated (Chou

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et al., 2002). With this technique called ‘laser-assisted direct imprint’ (LADI), a single excimer laser pulse melts a thin surface layer of silicon, and a mold is embossed into the resulting liquid layer. A variety of structures with resolution better than 10 nm have been imprinted into silicon using LADI, and the embossing time is less than 250 ns. The high resolution and speed of LADI, which was attributed to molten silicon’s low viscosity (one-third that of water), opened up a variety of applications and was extended to other materials and processing techniques. A similar nanopatterning technique, laser-assisted nanoimprint lithography (LAN), in which the polymer is melted by a single excimer laser pulse and then imprinted by a mold made of fused quartz has been used to pattern nanostructures in various polymer films on a Si or quartz substrate with high fidelity over the entire mold area (Xia et al., 2003). The entire imprint from melting the polymer to completion of the imprint was shown to be less than 500 ns. The mold has been used multiple times without cleaning between each imprint. LAN not only greatly shortens the imprint processing time, but also significantly reduces the heating and expansion of the substrate and mold, leading to better overlay alignment between the two. Tunable CO2 and non-tunable CO2 lasers were used for the irradiation of thin polypropylene films (Bormashenko et al., 2003). The wavelength of the IR radiation was adjusted in such a way that it coincided exactly with the absorbance peak in the spectrum of the polypropylene pattern. In this way, conditions of resonance absorption of IR radiation by polymer films were produced and the strong thermal effects in poly propylene produced under irradiation at the resonance wavelength were implemented in the NIL. With this method, called ‘infrared laser-assisted nanoimprint lithography’, the imprint was successfully performed into a S1805 photoresist film using a fused silica stamp with additional pulses from a Nd :YAG laser excitation with the wavelengths of 1064 nm (Grigaliunas et al., 2004), and into the thermoplastic polymer material mR-I 8020 using a prepatterned Si substrate, which is transparent for the CO2 laser irradiation (Grigaliunas et al., 2006). It was shown, that the thermoplastic resist mR-I 8020 could be successfully imprinted using the infrared continuous wave CO2 laser irradiation (λ = 10.6 mm) and the imprint quality may be improved by the selection of an appropriate film material which has a high threshold of ablation and a low softening point. A laser-assisted patterning procedure of conjugated polymer light-emitting diodes has been also developed using pulses from a 248 nm excimer laser (Lidzey et al., 2005).

10.3

Photo-assisted nanoimprinting

UV-NIL, also called step-and-flash imprint lithography (SFIL), is specifically suited to IC fabrication. Because a transparent template is used, the

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through-template optical alignment techniques used in optical lithography can be performed to print multilayer structures. The low imprint pressure also allows imprinting on fragile substrates, such as gallium arsenide and indium phosphide. Since UV-NIL is a room temperature process, the magnification and distortion errors in thermal NIL, due to differences in thermal expansion coefficients, are minimized. By using a monomeric fluid with a low viscosity, UV-NIL techniques avoid incomplete mold filling which is a problem for embossing polymers with a rigid molds. This lithographic technique is also insensitive to the effects of pattern density. The use of a liquid imprint material allows tailoring of the drop pattern to compensate for variations in pattern density and feature size. However, there are still challenges that must be clarified before the UV-NIL process can become a viable contender for fabrication of integrated circuits. First, the resolution of UV-NIL depends on the fabrication of a high resolution template. UV-NIL suffers from the potential for defect propagation through mold fouling and/or damage, which can result in high overall defectivity. UV-NIL templates are treated with a release layer to facilitate release of the template from the imprinted material and to act as a selfcleaning layer to prevent defect propagation through multiple imprints. Although UV-NIL was designed with specific consideration for the necessity of optical alignment, achievement of the tight alignment tolerances for 32 nm and 22 nm features is challenging. To achieve sub-15 nm overlay, tools must be further improved to control temperature, to correct for distortion errors between the wafer and template, and to design a suitable alignment method and optics. Finally, UV-NIL throughput needs to be improved. The templates for UV-NIL are usually made of quartz. Entire templates have also been fabricated out of diamond and sapphire substrates. Both diamond and sapphire offer superior mechanical properties compared with quartz, and sapphire is particularly hydrophobic. Diamond and sapphire templates can also withstand any corrosive and high temperature cleaning processes better than quartz, prolonging the lifetimes of the molds. However, a major drawback is that these substrates are substantially more expensive than quartz and require a higher initial cost for template manufacturing. In UV-NIL, stamps made from elastomeric materials, e.g., PDMS can also be applied. A simple and highly effective method for the replication of a soft mold based on an anodic aluminum oxide (AAO) membrane was developed (Zhou et al., 2009). The soft mold with nanopillar arrays comprised a toluene diluted PDMS layer supported by the soft PDMS. Hexagonally well-ordered arrays of holes of nanometer dimensions could be achieved over large areas by UV-NIL using the replicated soft PDMS mold. An important consideration in all nanomolding techniques is the lifetime of the mold. A mold for imprint lithography typically has a high density of

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nanoscale protrusion features on its surface. This effectively increases the total surface area that contacts the imprinted polymer, leading to strong adhesion of the imprinted polymer to the mold. This effect can easily be seen by the sticking of a resist material to a mold without any special treatment. Solutions to this problem are (i) incorporating an internal release agent into the resist formulation, (ii) applying a low surface energy coating to the mold to reduce its surface energy (or a combination of both approaches), and (iii) choosing a mold material with an intrinsically low surface energy (Guo, 2007). The most widely adopted approach is to form a self-assembled monolayer of a release agent on the mold surface. A release layer reduces the surface free energy of the mold and minimizes adhesion of crosslinked polymer to the mold. If the release layer fails, the cured polymer can adhere to the mold and foul its surface or break its features. Template fouling, caused by polymerized resist remaining adhered to the template after release, is a significant problem because damaged templates can cause yield loss. The first reported release layer was a fluorinated silane (Colburn et al., 1999) with a lifetime of less than 100 patterned substrates. With silicon oxide as stamp material, the main strategy is to improve the anti-adhesive properties of the stamp by coating with a fluorinated silane. The silanes exhibit strong covalent bonding and sufficient hydrophobicity. Fluorinated silanes are available with different carbon chain lengths and silane head groups. New surface treatments have been developed with a view to improving lifetime (Gates et al., 2005). Schift et al. (2005) found that the anti-adhesion properties of mold surfaces coated with fluorinated trichlorosilanes can be further improved by co-deposition of monochlorosilanes. This is because the introduction of monochlorosilanes helps to reduce the steric hindrance between the trichlorosilane molecules bound to the mold surface, resulting in a better molecular packing compared to coatings that use only trichlorosilane molecules. It has been shown experimentally that fluorosilane-treated molds can be used to imprint several hundred to a thousand times before their anti-adhesion properties degrade and a new coating is required. The commercially available silane, 1H,1H,2H,2H-perfluorodecyltrichlorosilane (CF3–(CF2)7–(CH2)2–SiCl3 or FDTS) was shown to provide good pattern transfer into the resist (Zhou et al., 2008). It was shown that a fluorosilane release layer applied to a UV-NIL template undergoes attack by acrylate, methacrylate, and vinyl ether UV-curable resist systems, pointing to its degradation being intrinsic to the chemistries involved. The successful use of diamond-like carbon (DLC) as a release layer for a methacrylate resist shows that a criterion of low reactivity rather than low energy is valid (Nakamatsu et al., 2006; Houle et al., 2007). It was shown that an ion-beam deposited diamond-like carbon release coating has

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both stability in a reactive environment and lower adhesion despite its higher surface energy. DLC has other important properties such as high wear resistance, high corrosion resistance, low coefficient of friction, good biocompatibility, and UV transparency. These interesting properties make DLC an ideal material for nanoimprint templates, not only as an anti-stick coating. It was shown that DLC films for imprint templates offer high wear resistance and robust anti-stick surface (Ramachandran et al., 2006). To optimize the anti-stick properties and the wear resistance, nanoimprint templates were fabricated from DLC films grown on silicon with an antiadhesion coating provided through fluorocarbon-based plasma treatment, which was found to form a teflon-like thin layer on the treated DLC surface (Schvartzman et al., 2008). The problem of the adhesion of the imprinted polymer to the mold can also be solved by incorporating an internal release agent into the resist formulation. This agent is a fluorinated surfactant to promote template release and to minimize the separation force applied to release the template from the imprinted material such that the imprint material adheres to the substrate, but not to the template. Apart from that, the resist must satisfy several important material characteristics. The formulation must be a low viscosity liquid at room temperature to enable ink-jet dispense, and to achieve the advantages of faster patterning and pattern filling at room temperature, it must also be photosensitive and not strongly absorbing at the exposure wavelength, and it should be quickly photocurable to achieve high throughput. The material must also be strong enough to avoid pattern collapse when the template is removed and must not exhibit large shrinkage during cure. At least 9% silicon is added to the resist to ensure sufficient etch selectivity during the oxygen-reactive ion etch step and to allow high aspect ratio features to be generated in the transfer layer. The material must also exhibit sufficient thermal resistance to withstand the etching temperatures. To achieve these properties, the imprint material is usually a mixture of components. The imprint formulation typically consists of a bulk polymerizable organic monomer, such as acrylate or vinyl ether (VE), a siliconcontaining or siloxane-containing monomer to provide oxygen-etch resistance, a crosslinking agent to provide mechanical strength and thermal stability to the imprint structure, a photoinitiator, and a fluorinated surfactant to promote template release (Costner et al., 2009). To create the desired asymmetry (strong adhesion to the substrate, weak adhesion to the mold), the resist was initially modified with a fluorine-based additive (1H,1H,2H,2Hperfluorooctyl-triethoxysilane) (Bender et al., 2002). Fluorine–carbon bonds are extremely stable and are used as anti-adhesive materials for a number of applications due to their low surface free energy. This additive migrates to the surface, a process which is thermodynamically controlled by the

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surface free energy and accelerated by the spin coating process. For a given blend system at equilibrium, in this case the base material and the perfluorinated silanes, the component with the lowest surface free energy will typically be enriched at the surface. The silicon-containing acrylates have several advantages, including low viscosity, fast curing rate, and commercial availability (Long et al., 2007). Although acrylate-based materials are attractive imprint materials owing to their fast curing rates and widespread availability, oxygen inhibition can lead to defect generation and lowering of the throughput. A possible solution is to use a material system that undergoes photopolymerization via an oxygen-insensitive mechanism, such as cationic polymerization. Monomer materials that undergo cationic polymerization include vinyl ethers, aldehydes, and epoxides. Epoxides have excellent material properties and are well known as structural polymers. For the majority of applications, the droplet dispensing method used to apply the liquid resist on a substrate in SFIL significantly limits the throughput of the nanopatterning process. The ability to spin-coat a uniform liquid resist onto a large-area substrate is highly desirable. Guo et al. have developed a UV-curable epoxysilicone material based on the cationic crosslinking of cycloaliphatic epoxies (Cheng et al., 2005; Guo, 2007). This resist combines a number of desired features for nanoimprinting. Because cationic polymerization is not prone to oxygen inhibition, as compared to the free radical polymerization of acrylate monomers, fewer defects are expected. The resist exhibits a very good dry etching resistance because of the high silicon content. Furthermore, its very low shrinkage after curing (only a fraction of the acrylate system) allows reliable patterning. In addition, with a suitable undercoating polymer a very uniform liquid precursor can be formed simply by spin-coating, which also allows other processes, such as lift-off, to be easily performed. The UV-curable liquid resist consists of a silicone-diepoxy monomer, a silicone crosslinking agent, and a photoacid generator. A typical liquid-resist formulation comprises diepoxy monomer (94%, w/w), crosslinkers (5%), and a photoacid generator (PAG) (1%). Organic solvents, such as propylene glycol monomethyl ether acetate (PGMEA), can be used to adjust the viscosity of the resist so that film thicknesses that range from 1 μm to 50 nm and below can be readily obtained. Photopolymerizable epoxy polyhedral oligomeric silsesquioxane monomers with variable aliphatic spacer have been prepared and cured under UV radiation (De Girolamo et al., 2008). Due to their organic/inorganic composition, these materials are promising candidates for microelectronic applications which require high thermal and mechanical stabilities as well as a low dielectric constant, for example, for the fabrication of electrical interconnects. It was pointed out that the use of a sensitizer in addition to

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the photoinitiator enhances the kinetic of the cationic polymerization of epoxy groups. A novel UV curable epoxy siloxane polymer has been used as the NIL resist to achieve features as small as 50 nm (Ye et al., 2010). The polymeric soft molds for the NIL were fabricated by casting toluene diluted PDMS on the hydrogensilsesquioxane (HSQ) hard mold. Another novel liquid photopolymerization resist was prepared for nanoimprint lithography on transparent flexible plastic substrates (Wu et al., 2009). The resist is a mixture of PMMA, MMA, methacylic acid (MAA), and two photo-initiators–2 isopropyl thioxanthone (ITX) and ethyl 4-(dimethylamino)benzoate (EDAB). The resist can be imprinted at room temperature with a pressure of 0.25 kg/cm2, and then exposed from the transparent substrate side using a broad band UV lamp to obtain nano- and microscale patterns. The liquid resist has low viscosity due to the liquid monomers, and low shrinkage due to the addition of PMMA as a binder. A new approach using a spin-on UV-sensitive hard mask underlayer material with terminal methacrylate groups has been developed successfully, in order to obtain high adhesion by radical polymerization between acrylate groups of the resist material and methacrylate groups of the underlayer during UV irradiation to avoid resist peeling and defect formation (Takei et al., 2010). The high adhesion obtained between the resist and underlayer was sufficient to reduce the generated resist peeling and improve the contamination problem when the template was removed from the resist after UV irradiation. The first underlayer material demonstrated indicated useful properties, such as 80 nm straight profiles on 20 nm thin residual thickness and nanoimprint patterning replication on 200 mm wafer. A novel nanoimprint process has been proposed based on UV-NIL combined with a novel imprint resist and often several advantages. The process (i) requires no extra temperature budget; (ii) is residue-free, thereby rendering etching obsolete; (iii) is time saving due to short curing times; and (iv) is eco-friendly due to a water-based lift-off (Auner et al., 2009). The imprint resist, which is called UV-NIL-ACMO, comprises an acryloyl morpholine monomer (ACMO) mixed with a photoinitiator blend (GENOCURE®) and highly diluted in chloroform, resulting in a low viscosity. Acryloyl morpholine provides removability by a lift-off process due to the lack of functional groups that could crosslink during polymerization, thereby preventing its solubility. Furthermore, the lift-off process can be accomplished with water as a cheap and eco-friendly solvent because of similarities in the polarity of the functional side group (morpholine) and the solvent water. Employing this method, functional submicron pentacenebased organic thin film transistors (OTFTs) can be fabricated. This UV-NIL technique works perfectly even when ultra-thin organic and hybrid films are used as gate dielectrics.

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297

Soft NIL

Soft lithography or nanotransfer printing is a type of nanoimprint lithography with a very weak press pressure. Microcontactprinting (μCP) has been developed primarily for PDMS stamps, although surface modifications of these stamps and other soft materials have been explored (Gates et al., 2005). Geissler and Xia (2004) classified five different printing schemes which rely on contact between the stamp and the substrate: (i) relief printing, (ii) intaglio printing, (iii) lithography printing, (iv) screen printing, and (v) xerography. The basic principles of intaglio and screen printing are closely related to molding and masked deposition. Lithography printing involves the use of a flat, chemically patterned stamp comprising areas that either accept or repel the ink. This concept is of crucial importance in the paper printing industry, but has not been frequently used for micropatterning. In xerography, the images are created by either charging or discharging selected regions on a substrate during contact. μCP is a form of relief printing. As briefly discussed above, in a typical procedure, the stamp is first inked by covering the patterned face with a solution of ink, dried, and then brought into contact with the surface of a substrate. μCP was first demonstrated for SAMs of alkanethiols on gold (Kumar and Whitesides, 1993), and was consequently extended to alkylsiloxanes on hydroxyl-terminated substrates, alkylphosphonic acids on aluminum oxide, and direct printing of chemical species such as catalysts or catalytic precursors, colloidal particles, dendrimers, organic reactants, conventional polymers, lipid bilayers, and proteins (Geissler and Xia, 2004). Self-assembly often provides routes to structures having greater order than can be reached in non-self-assembling structures. SAMs are usually prepared by immersing a substrate in the solution containing a ligand that is reactive toward the surface, or by exposing the substrate to the vapor of the reactive species. The formation of ordered SAMs on gold from alkanethiols is a relatively fast process. SAMs of alkanethiolates on gold exhibit many of the features that are most attractive about self-assembling systems: ease of preparation, density of defects low enough to be useful in many applications, good stability under ambient laboratory conditions, practicality in technological applications, and amenability to controlling interfacial properties (physical, chemical, electrochemical, and biochemical) of the system. Highly ordered SAMs of hexadecanethiolate on gold can be prepared by immersing a gold substrate in a solution of hexadecanethiol in ethanol for several minutes, and formation of SAMs during microcontact printing may occur in seconds (Xia and Whitesides, 1998). These factors ultimately determine the success of microcontact printing.

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Microcontact printing is one of the most useful techniques for generating patterns of functional organic surfaces over large areas. This methodology can tailor properties (wettability, biocompatibility, reactivity) of the surface, by transferring organic films that are only 1–2 nm thick (Gates et al., 2004). The flexibility of the stamp used in μCP allows conformal (i.e., molecular level) contact between the stamp and the substrate for a range of topologies, including planar and curved substrates. Feature sizes routinely achieved using μCP are above 100 nm. However, μCP has produced features with critical dimensions smaller than 50 nm on metal surfaces (Li et al., 2003; Gu et al., 2008). The flexibility of the PDMS stamp and the ability to achieve conformal, atomic-level contact between the stamp and the substrate are both advantageous for printing over large areas (>50 cm2) and on curved surfaces. Another variant of this technique is termed nanotransfer printing (nTP) (Loo et al., 2002). With nTP, a thin solid film (e.g. gold or aluminum) instead of liquid-based inks is transferred onto the surface of a substrate. The stamp can be either a soft or a hard material, such as PDMS or silicon. Cold welding (Tong, 2001) between two metal surfaces can also transfer the structured metal film. Techniques relying on non-covalent interactions between the metal film and the substrate, on condensation reactions between surface-bound silanols (Si–OH) and/or titanols (Ti–OH) have also been explored (Gates et al., 2005). A widely used approach for the production of metal nanostructures involves bilayer nanoimprint lithography. For instance, the use of a bilayer resist which relies upon the differential solubility between PMMA and PMMA-methacrylic acid copolymer) has been demonstrated to facilitate the metal lift-off step in imprinter fabrication (Faircloth et al., 2000). It was shown that the bilayer resist technology exhibits more uniform patterns and fewer missing features than similar metal nanoparticle arrays fabricated with single layer resist. A bilayer UV-NIL method that uses a monomerbased UV curable monomer resin was proposed as a method of imprinting at low temperature and pressure (Yang et al., 2006). To accomplish high fidelity patterns on a topographical substrate, the use of bilayer NIL, which involves the use of an easily removable under-layer and an imprinted pattern, was proposed. With this method, by etching the under-layer using oxygen RIE, it is possible to build the bilayer patterns for easy lift-off and to fabricate nano-sized metal patterns through this liftoff process. A new imprint method named nanoimprinting in metal/polymer bilayer structures (NIMB) has been demonstrated for patterning metal films with varied profiles (Chen et al., 2006). In contrast to conventional nanoimprint lithography, the patterned mold is directly imprinted in metal films rather than in polymer-based resists. Since direct imprint in metal films needs ultra-high pressure or temperature to form patterns, the direct imprint

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process was improved by using a sharp mold and an underlying soft pad layer to reduce the imprint pressure and temperature. It was shown that the imprint pressure can be reduced sufficiently that it is compatible with the conventional nanoimprint instrument. A room temperature nanoimprint lithography (RT-NIL) process using hydrogen silsesquioxane (HSQ) provides a simple fabrication method to realize sub-micron-sized patterns because it can be applied at room temperature. When the process is performed on a HSQ–PMMA bilayer resist, it is suitable for an additional metal sputtering and lift-off process because it features negative vertical profiles (Sung et al., 2007). By combining with a post-dry-etching process, this sequence is capable of patterning high resolution features in a resist with high aspect ratio (Tao et al., 2005). Conventional methods of patterning metallic structures include lift-off, wet chemical etching, RIE, and shadow evaporation. These patterning techniques require exposure to high temperatures, basic or acidic solutions, and/or organic solvents. Nanotransfer printing avoids harsh processing conditions and transfers nanostructures in one step. In another version of soft lithography, a solvent is used to soften a polymer material so that it can adapt to the shape of a PDMS mold (Xia and Whitesides, 1998). Solvent-assisted micromolding (SAMIM) shares operational principles with both replica molding and embossing, except that SAMIM uses a solvent instead of temperature to ‘soften’ the polymeric material. An elastomeric PDMS mold is wetted with a solvent suitable for dissolving the polymer, and is brought into contact with the surface of the polymer. The solvent dissolves (or swells) a thin layer of the polymer, and the resulting fluid, comprising polymer and solvent, conforms to the surface topology of the mold. While the mold is maintained in conformal contact with the substrate, the polymer solidifies as the solvent dissipates and evaporates to form relief structures with a pattern complementary to that on the surface of the mold. This process has been demonstrated for a number of polymers including Novolac photoresists, PS, PMMA, cellulose acetate, poly(vinylchloride) (PVC), and the precursors to conjugated organic polymers. The choice of solvent for a polymer determines the effectiveness and success of SAMIM. The solvent should rapidly dissolve or swell the surface of the polymer without swelling the PDMS mold, since this can distort the mold or destroy the conformal contact between the polymer and the mold. Swelling of the PDMS mold is minimal when solvents with a relatively high vapor pressure and a moderately high surface tension are used to ensure rapid evaporation of the excess solvent (e.g., methanol, ethanol, and acetone). The SAMIM process is simple, rapid, and does not require specialized equipment or a system for pressing the mold into the substrate. Another

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characteristic useful in nanofabrication is that it avoids cycling of the temperature of the sample and thus limits thermal oxidation or degradation of other system components. SAMIM is capable of replicating complex quasi3D relief structures over relatively large areas in a single step. Micromolding in capillaries (MIMIC) represents another nonphotolithographic method that forms complex microstructures on both planar and curved surfaces (Kim et al., 1995; Xia and Whitesides, 1998). In MIMIC, the PDMS mold is placed on the surface of a substrate and makes conformal contact with that surface. The relief structure in the mold forms a network of empty channels. When a low viscosity liquid pre-polymer is placed at the open ends of the network of channels, the liquid spontaneously fills the channels by capillary action. The flow of a liquid in a capillary occurs because of a pressure difference between two hydraulically connected regions of the liquid mass, and the direction of flow decreases this difference in pressure. After filling the channels and curing the pre-polymer into a solid, the PDMS mold is removed, and a network of polymeric material remains on the surface of the substrate. Microfabrication based on MIMIC is remarkable for its simplicity and its fidelity in transferring the patterns from the mold to the polymeric structures. However, it requires a hydraulically connected network of capillaries, and it suffers from slow rates of capillary filling, which limits MIMIC to relatively small areas, in comparison to SAMIM. MIMIC was developed based on pre-polymers having no solvents. The capability and feasibility of MIMIC have been demonstrated by the fabrication of patterned structures from a variety of liquid pre-polymers: poly urethane, polyacrylates, and epoxies. It has also been extended to systems with solvents (Xia and Whitesides, 1998). The solvents are evaporated after the solutions have filled the channels. However, the solvent must not swell the PDMS. The power of soft lithography to make structures for organic electronics was shown by Cavallini and co-workers (Serban et al., 2009). They employed MIMIC to make micrometer structures and lithographically controlled wetting to achieve submicron wires. A high resolution soft lithography technique – nanomolding in capillaries (NAMIC) – was demonstrated by Duan et al. (2010). Composite PDMS stamps with sub-100 nm features were fabricated by NIL to yield nanomolds for NAMIC. NAMIC was used to pattern different functional materials such as fluorescent dyes, proteins, nanoparticles, thermoplastic polymers, and conductive polymers at the nanometer scale over large areas. The major improvement of this method is the use of a hybrid nanomold with harder, more well-defined features, thus allowing accurate nanoscale surface patterning. It was shown that NAMIC is a simple, versatile, low-cost, and high-throughput nanopatterning tool.

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A method of CFL was introduced to combine the essential feature of imprint lithography, i.e., molding a polymer melt, with the prime element of microcontact printing, i.e., using an elastomeric mold (Suh et al., 2001). As a result, the advantage of imprint lithography over microcontact printing was retained in meeting the stringent pattern fidelity requirements for the fabrication of integrated circuits while eliminating the use of extremely high pressure that is needed in imprint lithography. Furthermore, the etching step needed to open the windows after imprinting could be eliminated. In CFL, an elastomeric mold is placed on a polymer that is spin-coated onto a substrate. The system is then heated above the glass transition temperature of the polymer. Capillary force allows the polymer melt to fill up the void space between the polymer and the mold. After cooling to ambient temperature, the mold is removed, thereby generating the negative replica of the mold pattern. The method has been demonstrated with PDMS as an elastomeric mold used for the fabrication of a PDMS master that has a planar surface with recessed or protruding patterns by casting PDMS against a complementary relief structure prepared by photolithography or the electron-beam method. For the polymer, a commercial PS and styrene– butadiene–styrene (SBS) block copolymer was used. Silicon wafer (100) was used as the substrate. However, other kinds of organic or inorganic substrate can be used if the surface is planar enough to allow conformal contact with the PDMS mold. The implementation of high resolution polymer templates fabricated by CFL was explored both in NIL and in the wet-etching of metals (Bruinink et al., 2006). Several different thermoplastic and UV-curable polymers and types of substrates were incorporated into the general CFL procedure to meet the diverging requirements of these two applications. It was shown that CFL is a convenient and inexpensive technique for fabricating functional, high resolution polymer templates with high fidelity and excellent uniformity, without the need of advanced lithographic techniques.The incorporation of UV curable mr-L6000 polymer in the CFL procedure has allowed the fabrication of high stability NIL molds, by UV crosslinking.

10.5

Extensions of soft NIL

In soft lithography, the press pressure is still necessary, even if it is very weak. On the other hand, there are methods where the pressing process is not very important and the surface reaction becomes dominant. One of these methods is chemical nanoimprint (Namatsu et al., 2007). The typical approach uses an electrochemical reaction to transfer a pattern. Another technique is surface charge lithography. Figure 10.3 shows the difference between conventional nanoimprint, electrochemical nanoimprint, and surface charge lithography. Once the surfaces of the mold and that of the

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(b)

(c) V

lon beam

V

Oxide mask

V

Surface charge mask

10.3 Schematic process diagrams of (a) conventional physical nanoimprint; (b) chemical nanoimprint using anodic oxidation; and (c) surface charge lithography.

substrate (i. e. silicon) are adjacent or in contact with each other in the electrochemical, a voltage is applied. The strong electric flux from the convex parts of the mold to the substrate results in the oxidation of the silicon surface corresponding to the convex parts of the mold with the moisture present between the mold and the substrate. Consequently, the substrate is subjected to etching resulting in the formation of the pattern determined by the oxide mask formed during the previous electrochemical oxidation. In surface charge lithography (Fig. 10.3c), a surface charge mask is formed on the surface of the substrate by ion irradiation. The main difference between the conventional physical nanoimprint and the electrochemical nanoimprint processes is the presence or absence of resist films. In the conventional nanoimprint, the mold has to be imprinted into the resist film and then released from it, because the resist film is the processing object. The resist process may cause defective patterns that result from pushing and releasing when the resist does not enter or stick to the mold patterns. On the other hand, with electrochemical nanoimprint process, these problems are avoided, since no resist is used. Therefore, there is no adhesion of resist to mold, no resist flow during pressing, no need for a mold with high aspect ratio, and no need to raise or lower mold temperature. As a result, one obtains fewer defects, less mold damage, and improved throughput. In contrast to conventional nanoimprint molds, good conductivity is required for a mold suitable for electrochemical nanoimprint lithography. A mold structure with a poly-SiC film formed on a sintered SiC substrate

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was devised for this purpose (Namatsu et al., 2007). It was applied to a silicon substrate in conjunction with etching the substrate in an aqueous solution of potassium hydroxide (KOH) employing the SiO2 patterns formed during the electrochemical reaction as an etching mask. A patterning of silicon substrate prepared by EB lithography and an etching process in conductive Silicon was also used as a nanoelectrode (mold) (Yokoo, 2003). This nanoelectrode was used for the anodic oxidation of a Si substrate. When the film to be etched is not silicon but a metal, the process is similar if the surface of the metal is easily oxidized. When the film to be etched cannot be oxidized, it is necessary to form a hard mask on the layer. For example, the hard mask may be composed of a silicon layer and a metal layer. The pattern is transferred to the silicon layer by applying a voltage between the mold and the metal layer (Namatsu et al., 2007). A solid-state electrochemical nanoimprint process for direct patterning of metallic nanostructures has been proposed (Hsu et al., 2007). It uses a patterned solid electrolyte or superionic conductor (such as silver sulfide) as a stamp and etches a metallic film by an electrochemical reaction. The process has been conducted in an ambient environment. Since it does not involve the use of liquids, it displays potential for single-step, high throughput, large-area manufacturing of metallic nanostructures. More generally, electrical microcontact printing uses a flexible electrode to pattern a thin film of a material that is an electret (i.e., that accepts and maintains an electrostatic potential) through injecting and trapping charges (Gates et al., 2004, 2005). The electrode can be a PDMS stamp coated with a film of chromium (adhesion layer) and of gold (electrode material). This flexible electrode was brought into contact with a thin dielectric film (the electret, a material such as PMMA) supported on a second electrode (typically n-doped silicon). A voltage pulse (10–30 V) was applied between the two electrodes for 10 s with current densities of 10 mA/cm2. Charge remained in the electret where the flexible electrode contacted the dielectric film. A number of methods exist for charging electrets, such as corona discharge and tribocharging (Gates et al., 2005). The stability of the patterned charge (for periods greater than months) is unexpectedly high. In surface charge lithography, the surface charge is induced by ion irradiation. It was shown that host defects introduced at the surface of GaN epilayers by 2-keV-argon ion beam are resistant to photoelectrochemical etching in aqueous solution of KOH, due to trapped negative charge (Tiginyanu et al., 2005). The spatial distribution of surface defects and related negative charge was designed as a lithographic mask for the purpose of GaN mesostructuring. To demonstrate the possibility for using surface defects as lithographic mask, selected areas of GaN layers were irradiated by 2-keV-Ar+ ions. Figure 10.4 illustrates a set of GaN mesostructures

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(b)

20 μm

10 μm

100 μm

10.4 SEM images taken from GaN layers subjected to PEC etching after selected areas were treated by Ar ion beam. Inset (a) shows a set of GaN mesostructures after PEC etching for 10 min, and (b) illustrates the morphology of a GaN mesostructure after PEC etching for 1 hour under intense stirring of the electrolyte. (Reprinted from Tiginyanu IM, Popa V and Volciuc O (2005), ‘Surface-charge lithography for GaN microstructuring based on photoelectrochemical etching techniques’, Applied Physics Letters, 86 (17), 174102. Copyright (2005) with permission from AIP)

fabricated by ion beam treatment followed by photochemical (PEC) etching of GaN epilayers. The surface charge mask can also be produced by low dose FIB treatment. The maskless approach based on ultra-fast direct writing of surface negative charge was used to fabricate GaN nanowalls and nanowires with lateral dimensions as small as 100 nm (Popa et al., 2008; Tiginyanu et al., 2009). Compared with commonly used lithography masks and/or FIB etching approaches for patterning GaN, the surface charge lithography enables one to fabricate high aspect ratio micro- and nanostructures and mitigates the need for additional mask layers on the surface prior to etching. Moreover, it is much faster than FIB etching alone, thus reducing the ion exposure of material and therefore reducing ion beam damage. Figure 10.5 illustrates GaN structures with different thicknesses and spaces between them obtained by FIB direct ‘writing’ followed by PEC etching for 60 min. The FIB bitmap pattern for the parallel rectangular structures is shown in the lower part of Fig. 10.5. Comparing the FIB bitmap pattern with the actual morphology of fabricated rectangular structures one can claim the following. First, using surface charge lithography (SCL)

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2 μm

A B C D E F

G 20 μm

A

B

C D

E

F

G

10.5 GaN-based mesostructures fabricated by FIB treatment followed by PEC etching for 60 min. The insert shows the magnified view of one wall. The lower part shows the exposure pattern of FIB treatment corresponding to rectangular mesostructures (1 pixel ∼ 30 nm). (Reprinted from Tiginyanu IM, Popa V, Sarua A, Heard PJ, Volciuc O and Kuball M (2009), ‘Surface charge lithography for GaN micro- and nanostructuring’, Proceedings SPIE, 7216, 72160Y. Copyright (2009) with permission from Society of Photo-Optical Instrumentation Engineers (SPIE))

techniques it is possible to etch voids in between structures as narrow as 200 nm (this is just the spatial separation between the remaining two structures in Fig. 10.5). Second, it is possible to reach fine spatial modulation of the mesostructure walls by proper design of the FIB pattern (see insert in Fig. 10.5). Third, there is a threshold lateral dimension of the FIB nontreated regions adjacent to FIB-treated areas for the PEC etching to occur. The top shielding region proves to be vulnerable to prolonged PEC etching. This vulnerability was used to transform the thin GaN walls in inplane nanowires. The approach relies on two distinct etching processes: the

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first is the PEC etching of non-treated areas leading to the formation of nanowalls, while the second starts at a later time, after the initiation of the dissolution of the top shielding area, the latter process resulting in the formation of nanowires. Note that for structures thinner than twice the extension of the surface depletion layer, the second process will take place providing they are directly connected to relatively large mesostructures. This is because the PEC current flows along the top UV illuminated surface of the nanowall closing a circuit through merging mesostructures. An isolated nanowall cannot be photoelectrochemically etched from the top due to its low conductivity caused by overlapping surface depletion layers. Figure 10.6 illustrates a GaN nanowire connecting two triangular shaped mesostructures. Connected mesostructures are desirable for specific applications, e.g., they can be used as platforms for the fabrication of Ohmic contacts. Thus, SCL proves to be an efficient tool for manufacturing in-plane GaN nanowires in a controlled fashion, without the use of any lithographic techniques. An important point is that in this case the top GaN areas damaged by the FIB are actually removed, and both the remaining nanowire and merging mesostructures contain a virgin GaN epilayer grown by metal organic chemical vapor deposition (MOCVD) on SiC substrate. Therefore it offers the possibility to fabricate in-plane networks of nanowires for various electronic applications in a controlled fashion. The surface charge mask can be also written by mechanical means with scanning probe techniques (Tiginyanu et al., 2009). Scanning probe techniques can also be used for patterning charge in electrets with electrical microcontact printing, or for local anodic oxidation in electrochemical

(a)

(b)

20 μm

2 μm

10.6 GaN nanowire merging triangular mesostructures: (a) top view; (b) magnified lateral view. (Reprinted from Tiginyanu IM, Popa V, Sarua A, Heard PJ, Volciuc O and Kuball M (2009), ‘Surface charge lithography for GaN micro- and nanostructuring’, Proceedings SPIE, 7216, 72160Y. Copyright (2009) with permission from Society of Photo-Optical Instrumentation Engineers (SPIE))

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lithography. However, the throughput is too low to perform large area patterning, even if a multi-needle or patterned-needle cantilever is used. Photocatalytic lithography is a kind of chemical nanoimprint, whereby a SAM is patterned by oxidation using UV irradiation and a titanium oxide mold (Lee and Sung, 2004). It was found that the monolayers are rapidly and homogeneously decomposed on a TiO2 thin film under UV irradiation in air through the photo-oxidation of the alkyl chains. The photocatalytic lithography of alkylsiloxane SAMs was applied in combination with selective deposition of thin films using atomic layer deposition. This approach consists of three key steps. First, the alkylsiloxane SAMs were formed by immersing Si substrate in alkyltrichlorosilane solution. Second, photocatalytic lithography using a quartz plate coated with patterned TiO2 thin films was done to prepare patterned SAMs of alkylsiloxane on the Si substrate. Third, ZrO2 thin films were selectively deposited onto the SAMs-patterned Si substrate by atomic layer deposition.

10.6

Scanning probe lithography (SPL)

Writing with a rigid stylus, known also as micromachining, involves mechanical displacement or modification of a material on the surface of a substrate. The process generally requires a direct contact between a surface of a substrate and a rigid stylus that is moved across the surface to write a pattern on the substrate. The writing of high-resolution patterns is accomplished by scanning probe techniques. SPL provides a versatile set of tools for both manipulating and imaging the topography of a surface with atomic-scale resolution. Although scanning probes were originally designed to provide high resolution images of surfaces, their lithographic capability was demonstrated in a set of experiments. The most important SPL techniques include scanning tunneling microscopy (STM), atomic force microscopy (AFM), scanning electrochemical microscopes (SECM), and near-field scanning optical microscopy (NSOM). STM and SECM involve writing with an electric field. An auxiliary electrode is moved above a conductive surface which serves as working electrode in this process. The applied electromagnetic field can induce a variety of physicochemical changes to the species on the surface. These changes can derive from several processes such as charging, increasing the current flow (Ohmic heating), redox reactions occurring at the tip or on the surface of the sample. SPL can involve different processes for patterning, such as depositing clusters of atoms or molecules onto a surface in a well-defined pattern (the add-on process), selective removal of material from a surface by force induced patterning, and modifying a surface chemically (Gates et al., 2005). Add-on writing is attractive when one needs to pattern a material that is incompatible with the resist, image development, or etching process

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(Geissler and Xia, 2004). A direct add-on process can also provide excellent control over the surface chemistry of a substrate, and allows for the fabrication of systems with high chemical/functional complexity without severe cross-contamination. Dip-pen nanolithography (DPN) is the most widely used add-on process. An AFM tip is ‘inked’ with a solution of the material to be transferred to the surface. The material adsorbed onto the AFM tip transfers to the surface in an arbitrary pattern ‘written’ with the scanning probe. This technique has been applied to pattern nanostructures of metals such as Au, Pd, and Ag, alkanethiol monolayers on gold, colloidal particles of sols, inorganic metal salts, and polymers, as well as for binding oligonucelotides, proteins, and viruses (Geissler and Xia, 2004; Gates et al., 2005). Other variants of add-on process include laser-induced chemical vapor deposition (LCVD), FIB deposition, and ink-jet printing (IJP). LCVD has been widely used as a method of patterning solid substrates, since it is a fast and reliable way to deposit a variety of material on both planar and nonplanar surfaces (Geissler and Xia, 2004). Two processes are involved in the LCVD: photolytic decomposition or pyrolytic decomposition. In photolytic decomposition, the photons adsorption is accompanied by cleavage of chemical bonds, while in pyrolytic decomposition the precursor compound is thermally dissected by heating. Laser-assisted deposition of metal complexes from solution phase, laser-enhanced electro- and electroless plating, and photothermal decomposition of metal-doped polymer thin films have been demonstrated (Geissler and Xia, 2004). FIB-based deposition is a most commonly used techniques for repairing the defects on photomasks or integrated circuits. In IJP, small volumes of liquid ink from a reservoir are ejected through a nozzle and dispensed onto the substrate. IJP is a contactfree technique, it is capable of providing high throughput when a large number of nozzles are used in parallel, it allows simultaneous printing of different materials delivered from multiple nozzles, and it provides good alignment capabilities. The selective removal of material with an AFM tip in contact with the surface is referred to as nanoshaving. This process removes SAMs from a surface in a well-defined pattern. On the other hand, a second material (e.g., SAMs or nanoparticles) can replace the removed film. This substitution lithography is a convenient method for patterning multiple types of SAMs on a surface. Regarding the chemical modification of a surface, it was demonstrated with conductive AFM or STM tips which induced localized oxidation of a surface (metal, semiconductor, or SAM), as well as with NSOM used for chemical modification of a surface by photochemical oxidation (Gates et al., 2005). AFM has been the most widely used technique with typical approaches including the use of an AFM tip to scratch nanostructure in soft materials, to expose thin films of resist, to induce and/or enhance

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oxidation of H-terminated Si-(100), to change the headgroups or packing density of organic monolayers catalytically, and to ‘write’ 30 nm patterns of alkanethiols on gold (Xia et al., 1999). The system based on H-terminated Si(100) is most important for applications in microelectronics since almost all silicon devices are fabricated from this type of wafer. STM tips have also been widely used to alter the structure or order of organic monolayers, to oxidize hydrogen-terminated silicon, to induce phase transition in a solid material, and to manipulate atoms or molecules. Magnetic field can also be used for writing. Magnetic writing (and reading) is central to information storage technology (Geissler and Xia, 2004). In this method, the writing element is an inductive device on a recording head, which is used to magnetize small areas of a magnetic medium (e.g., iron oxide particles, or polycrystalline alloys composed of Cr, Co, Pt, etc.). The reversibility of the induced changes is an advantage of magnetic writing, since it is a purely physical process. The read-back of the recorder signal can be achieved using a magneto-resistive sensor hosted on the same head. Advantages of SPL methods include resolution that, for AFM and STM methods, approaches the atomic level, the ability to generate features with nearly arbitrary geometries, and the capability to pattern over non-planar surface topography. The commercial availability of AFM, STM, and NSOM makes these tools convenient for nanofabrication. These instruments are also capable of nanoscale registration. However, this approach to writing nanoscale patterns with a single tip is a serial process and is fundamentally slow which results in a low sample throughput. One approach to overcome this shortcoming is the incorporation of integrated arrays of tips that can write in parallel by using new designs for tips to make this technique semiparallel. This approach will certainly increase the throughput of SPL, but will also significantly increase its complexity and cost. The pattern can also be written with an electric field or current. An auxiliary electrode is moved above a conductive surface which serves as working electrode in this process. The applied electromagnetic field can induce a variety of physicochemical changes to the species on the surface. These changes can derive from several processes such as charging, increasing the current flow (Ohmic heating), redox reactions occurring at the tip or on the surface of the sample.

10.7

Edge lithography

Edge lithography comprises a class of techniques that use topographic edges in the fabrication of nanoscale features. These approaches contribute to size reduction, and are attractive for fabrication of high-resolution lines (wires or trenches) or other simple patterns such as dots or rings of various geometries. Edge lithographic techniques are usually classified into two

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categories: those using topography directed pattern transfer, and those using the process of transforming a feature that is thin in the vertical direction into a feature that is thin in the lateral direction (Gates et al., 2005). The latter are obtained by cutting or cleaving a substrate to expose an edge with nanoscale features. Topography directed pattern transfer is accomplished by several methods such as phase-shifting photolithography, decoration of step-edges, depositing material or etching at edge-defined defects in SAMs, and undercutting at edges. Near-field phase-shifting photolithography derives from conventional contact-mode lithography, but uses a conformal mask (made of an elastomer PDMS) to achieve a better contact with the resist film. The vertical edges of the transparent topographically patterned mask induce abrupt changes in the phase of incident collimated light to create narrow regions of constructive and destructive interference. As a result, dark or bright spots of incident light are projected onto the surface of a photoresist. Optimally, the light should have a phase-shift of p at the photoresist–mask interface. By using light with a wavelength of 248 nm, lines as narrow as 50 nm have been fabricated in a positive-tone resist. Patterns of photoresist can serve as etch masks to pattern nanoscale rings in a Pd film on a Si substrate or Al on CaF2. Photoresist patterns were also used as etch masks for producing uniform, single-crystalline silicon nanostructures of a variety of shapes (e.g., wires and loops) with well-defined widths down to 40 nm (Gates et al., 2004). Components of devices fabricated by phase shifting edge lithography include frequency-selective optical filters, optical polarizers, and gates for organic transistors having dimensions as small as 100 nm (Gates et al., 2005). The decoration of step-edges can generate continuous nanowires. For example, oxides (e.g., MoOx, MnO2, Cu2O, and Fe2O3), semiconductors (e.g., MoS2 and Bi2Te3), and metals (e.g., Ag, Pd, Cu, and Au) have been electrodeposited with lateral dimensions down to 15 nm at the step-edges of highly oriented pyrolytic graphite (HOPG) (Gates et al., 2004, 2005). The growth of metal nanostructures by physical vapor deposition can also be directed by step-edges on single-crystalline surfaces. Depositing material or etching at edge-defined defects in SAMs is another strategy for patterning nanostructures. For example, SAMs form polycrystalline lattices on a planar metallic surface but remain disordered at the edges of this surface. Sharp metal corners within a topographically patterned metal substrate prevent the formation of well-ordered SAMs and expose the underlying metal at these edges. Selectively etching the exposed metal transfers the outline of the patterned metallic topography into the underlying film with line widths as small as 50 nm (Aizenberg et al., 1998; Gates et al., 2005). Copper nanowires with lateral dimensions as small as 70 nm were produced by electrodeposition on edges made by engineering

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defects in SAMs. Photolithography can pattern regular arrays of topographic features, and electrodeposition on edges within this well-defined surface can generate aligned nanowires (Gates et al., 2005). Another approach to the use of topographic edges for the fabrication of nanostructures is the controlled undercutting of topographic features by isotropic wet etching, followed by deposition of a thin film. Well-defined trenches, with lateral dimensions as small as 50 nm, have been patterned by this method. Shadow evaporation of metal onto the sidewalls of topographic features, followed by the selective etching of the substrate, generates narrow, vertical structures. This method has been used to fabricate an array of 30 nm wide lines of silica. 10 nm wide vertical structures have also been produced by depositing a low temperature oxide uniformly over topographic features and etching the substrate by RIE (Gates et al., 2005 and references therein). In the second category of edge lithography related to generating nanostructures by exposing the edge of a thin film, the edge can be produced by fracturing thin films, by sectioning encapsulated thin films with a microtome, and by reorientation of metal capped posts (Gates et al., 2005). There are many ways to grow thin films with well-controlled thickness between 1 and 50 nm over large areas of surface. The cross-sections of multilayer films prepared using molecular beam epitaxy (MBE) have been exploited as templates to pattern simple quantum structures such as quantum wires and quantum dots (Gates et al., 2004). An MBE-grown substrate consisting of alternating layers of AlGaAs and GaAs was also used to fabricate an array of field-effect transistors (FETs) with 20 nm gate lengths. The edge of a multilayered substrate that has been fractured can reveal an array of narrowly spaced grooves by selective etching of one of the components, and can template the formation of parallel nanowires by physical vapor deposition. The width of the groove, corresponding to the original thickness of the etched film, determines the spacing between the nanowires. Modifying the thickness of each layer of the substrate will change the spacing between the nanowires and the width of the nanowires. Platinum nanowires were fabricated with diameters down to 8 nm and center-to-center distances of 16 nm (Gates et al., 2005).

10.8

NIL for three-dimensional (3D) patterning

3D structuring requires control of both lateral and vertical dimensions of the components. Photolithography can only generate one-dimensional (1D) or two-dimensional (2D) patterns. The fabrication of 3D structures usually requires more than one processing step and relies on a layer-by-layer fabrication with accurate alignment between the layers. However, the production cost exponentially increases with the number of layers, and defects

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accumulate as the number of layers increases. The fabrication of 3D structures by photolithography remains still a great challenge. NIL provides an alternative approach to attaining this goal. We will shortly describe some other approaches for 3D structuring before the overview of the NIL. While conventional photolithography requires the use of binary masks for multiple exposures with alignment between steps, photolithography with grayscale masks yields 3D structures in a single exposure. In this version of photolithography, the mask encodes the final shape of a structure in the form of an optical density contour map that determines the illumination intensity during the exposure of a resist or another sensitive material (Geissler and Xia, 2004). Holographic patterning which relies on the interference between intersecting laser beams is another approach for 3D structuring. Complex patterns can be produced by intersecting more than two laser beams or by using multiple sequential exposures. 3D periodic lattices are generated when four coherent laser beams are focused onto the same spot, and the exposed regions are removed by selective dissolution. This technique is suitable for the fabrication of 3D photonic crystals. Writing with focused laser or electron beams can also be used for obtaining 3D structures through the transformation of a 2D pattern into a 3D structure by varying the exposure dose during scanning. The add-on and layer-by-layer fabrication of 3D structures is based on the creation of a 2D pattern first, which serves as a template for subsequent deposition steps. The repetition of patterning and deposition layer-by-layer results in the production of 3D structures with desired shape. This approach has been demonstrated for contact printing with elastomeric stamps, and for IJP and robotic deposition in conjunction with different types of ink formulations that include colloidal fluids and suspensions, ceramic powders, or polymer-based binder solutions imprinted into compact payers of ceramic powder (Geissler and Xia, 2004 and references therein). There is a strong need for 3D nanoscale patterning technology for various optical devices and dual damascene processes in the next generation. A breakthrough in this field has been made by NIL because of its low cost and process simplicity. Layer-by-layer techniques have also been developed based on NIL and polymer bonding. For successful multilayer fabrication, it is very important to control the adhesion between the two polymer layers and between mold and polymer. Two techniques, bonding with a thin adhesive layer and direct thermal bonding near glass transition temperature, have been proposed to achieve good bonding between two polymer layers while maintaining the structural integrity of the bottom polymer layers (Park et al., 2007). Microtransfer molding was demonstrated to be suitable for forming complex, 3D microstructures of organic polymers and ceramics (Zhao et al.,

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1996). A heat-curable liquid epoxy (F109CLR) in the surface relief structure of a PDMS mold was partially cured at 65 °C for 25 min. The pre-cure of the pre-polymer increased its viscosity, and thus greatly reduced the possibility that the pre-polymer in the grooves of the mold would be drawn into gaps in the microstructure of the first layer by capillarity. A substrate, the surface of which had already been patterned with one layer of a microstructure, was placed upside down onto the mold with a pre-cured, filled relief structure. The whole system was then fully cured at 25 °C for 24 h. The PDMS mold was peeled away and a two-layer microstructure was obtained. The process was repeated to make the third layer. A 3D nanostructure was also imprinted using a 3D mold (Sun et al., 1998), and more complex 3D nanostructures such as sealed cavities were obtained by combining simple 2D geometries from two molds (Kong et al., 2004; Low et al., 2006). The careful selection of mold geometries in this method constitutes a simplified and efficient approach toward building up desirable 3D structures without resorting to the use of a sacrificial process or components. 3D structures fabricated for a variety of specific applications were presented using both thermoplastic and crosslinked polymer materials. 3D nanostructures, in particular with sub-40 nm metal T-gates and an air-bridge structure, have been patterned with NIL in a single-step imprint in polymer and metal by lift-off (Li et al., 2001). For this purpose, a method based on EBL and RIE was developed to fabricate NIL molds with 3D protrusions. Nanometer-order 3D imprint molds have also been fabricated using acceleration voltage modulation electron beam (EB) direct writing, in which spin-on-glass (SOG) is used as an EB resist whose depth is controlled by changing the EB acceleration voltage (Taniguchi et al., 2004). The exposed SOG area and depth were developed with 1 EB exposure using buffered hydrofluoric acid (BHF), yielding a 3D SOG mold. Two acceleration voltage changes were used, i.e., changing the EB gun bias and changing the substrate voltage. A 3D sub-100 nm nanoimprint mold has been fabricated by using control-of-acceleration-voltage EBL with inorganic resist, and by using UV-NIL, a replicated pattern was obtained that approximately corresponded to the fabricated mold (Unno et al., 2007). The process of fabrication of 3D gold pattern in one shot on the PET substrate by using EBL is illustrated in Fig. 10.7. The fabrication process of a 3D metal nanoimprint mold involves three major steps (Unno et al., 2009). First, a positive tone EB resist is spin-coated on a cleaned Si substrate and cured to form a 500 nm film. Second, the sample is delineated by EB changing acceleration voltage. Then the EB-exposed area on the SOG film is developed. Finally, a Cr layer having a thickness of around 10–20 nm is deposited on a fabricated 3D mold. After coating the mold with Cr, the surface of Cr is oxidized to Cr2O3. Next, gold is deposited on the Cr layer, after which the hot plate is heated, and the

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80 °C 30 min. @ Hot plate Metal film gold

500 nm

Spin on glass Si (1) Contact

Release layer Cr2O3 (2) Applied heat and transfer

(3) Release

10.7 Technological steps for fabrication of three-dimensional gold pattern on the polyethylene terephthalate substrate by using EBL. (Reprinted from Unno N, Taniguchi J, Ide S, Ishikawa S, Ootsuka Y, Yamabe K and Kanbara T (2009), ‘Three dimensional metal nanoimprint technique for electrode and electric probe’, Journal of Physics: Conference Series, 191, 012014. Copyright (2009) with permission from IOP)

PET substrate is placed on the hard mold for 30 min. After that, the PET substrate is removed, and the gold pattern is transferred onto the PET substrate. The fabrication of ordered 3D hierarchical nanostructures by NIL has been demonstrated through exploiting the properties of imprinted polymers (Zhang and Low, 2006). The hierarchical structures were obtained by a series of sequential imprinting steps, where smaller structures are imprinted on top of larger imprinted structures. Higher order hierarchy was achieved by sequentially adding imprinting steps. An important feature of this fabrication technique is that the subsequent imprinting is carried out at a temperature below the glass transition temperature of the polymer film and below the imprinting temperature of the preceding imprint without the assistance of any solvents or plasticizers. This technique is suitable for the production of various formats of hierarchical structures by varying the mold geometries and mold orientations in the sequential imprinting. Reverse contact UV-NIL is also an effective tool for the fabrication of 3D multilayered nanostructures (Kehagias et al., 2006). This technique is a combination of reverse NIL and contact ultraviolet lithography. In this process, a UV crosslinkable polymer and a thermoplastic polymer are spincoated onto a patterned hybrid metal-quartz stamp. These thin polymer films are then transferred from the stamp to the substrate by contact at a suitable temperature and pressure. The whole assembly is then exposed to UV light. After separation of the stamp and the substrate, the unexposed polymer areas are rinsed away with acetone, leaving behind the negative features of the original stamp with no residual layer.

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Combined nanoimprint approaches

In conventional UV lithography, the structure resolution is limited by the wavelength of the light, and generation of structures in the 100 nm range is complicated and expensive. On the other hand, NIL works effectively for nanoscale features, but experiences difficulties when replicating larger features due to material transport problems. In many cases, lithographic techniques capable of producing both large and small features in various combinations and with various pattern densities are required. Evidently, a combination of these two techniques, i.e. a mix-and-match lithography approach, should make it possible to generate nanometer patterns along with micrometer patterns within one layer. However, the use of two separate lithography steps will result in an increased complexity and will require pattern registration between the two separate steps. Apart from that, in a combination of the two techniques it is necessary that the processing conditions chosen for the imprint do not hamper a subsequent UV lithography step. This may be a problem, since an adequate polymer flow during imprint requires temperatures of up to 100 °C above the glass transition, where the photoactive resist system may suffer from e.g. decomposition or outdiffusion of the light-sensitive components or from partial crosslinking. A combination of NIL and UV lithography was proposed using polymers which are both imprintable and photosensitive (Pfeiffer et al., 2001). Based on an epoxy-type negative tone, chemically amplified resist system patterns were easily obtained using both techniques. In the first step, a pattern relief was produced through imprinting. In the second step, this relief was UV exposed in a contact printer and then developed. In addition to two-step profiles, three-step profiles can also be obtained, when the residual layer of NIL is not removed. This offers new routes for device design, in particular when in addition a chemical functionality is implemented in the resist. To solve the problem of defect formation and pattern failures resulting from the high viscosity of the polymer melt and the varied pattern densities on the mold, a technique was developed by introducing a hybrid mask concept and combining NIL with photolithography (CNP) (Cheng and Guo, 2004a, b). The hybrid mold is made of a UV-transparent material and acts as both an NIL mold and as a photolithography mask. Protrusions are made on the mold for imprinting nanoscale features, while metal pads are embedded into the mold and serve as a photolithography metal mask to replicate the large patterns. In the CNP process, the hybrid mold is first imprinted into the resist layer by pressure, and subsequently the entire mold–substrate assembly is exposed to UV radiation (Guo, 2007). After the hybrid mold and the substrate are separated, the substrate is immersed in a developer solution to remove unexposed resist (i.e., resist that was blocked by the metal pads). After development, both large and nanoscale patterns

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are created in the polymer resist in one step. The fabrication of nanoelectrode structures with drastically different length scales, from 150 μm down to tens of nanometers, has been demonstrated with this technique, and these structures were used in making nanoscale organic thin-film transistors. Potential and limitations of a T-NIL/UVL hybrid process have been recently discussed (Scheer et al., 2010). X-ray lithography (XRL) can be used instead of photolithography in the combined mix-and-match lithography. This combination has been used for the fabrication of unconventional 3D polymer structures (Tormen et al., 2004). The use of XRL for structuring a pre-patterned resist by NIL gives rise to high resolution high aspect ratio structures whose overall profile is enveloped by the original 3D imprinted profile. The technological potential of this method has been demonstrated by patterning several different types of structures with XRL on an hexagonal array of hemispheres previously obtained by nanoimprinting. It is suggested that this technique should allow upscaling the fabrication of complex 3D nano- or micro-objects to a mass-production level, since both NIL and XRL are parallel patterning techniques. Apart from combination of photolithography and NIL lithography, significant improvement can be achieved by combining different NIL techniques. Hybrid patterning by thermal and UV-NIL was used to fabricate micro and nanomixed structures (Okuda et al., 2007). The SU-8 resist was thermally imprinted using a quartz mold with fine nanostructures and micro Cr mask patterns. After the thermal nanoimprint, UV light was exposed to the resist through the mold. Then, the mold was released and the resist was developed to fabricate microstructures. Using this process, nanodots having 200 nm feature sizes were successfully demonstrated on the microgratings with 40 μm width and 20 μm height. Also, fabrication of a nanocorn array on the bottom of the deep microwell was demonstrated using Ni mold replicated from the resist master structure fabricated by the hybrid NIL. Direct thermal-UV nanoimprinting of an organometallic hybrid film has been demonstrated to fabricate nanoscale features into a novel organic– inorganic solution containing selected metals (Han et al., 2010). With this combination, the film can be patterned at low temperature and pressure, and requires only a short processing time. The low processing temperature enables nanoimprinting applications on a wide array of substrates, which further expands the applications of the process. The thermal stability of the organometallic hybrid film is significantly improved by UV irradiation and oxygen plasma treatment, and provides potential applications for the film as a functional part of a final NIL-created device expected to operate at relatively high temperatures. Another interesting combination is capillary force lithography combined with replica molding which has been demonstrated for the fabrication of

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high-resolution stamps for soft lithography (Bruinink et al., 2004). This is a two-stage procedure in which CFL is carried out first to fabricate robust high resolution masters, consisting of submicrometer-size polymer edge structures that are subsequently replicated by replica molding (REM) to generate PDMS stamps for high resolution soft lithography. These stamps have been used in different μCP experiments on gold. In another approach, a combination of NIL with contact printing was used for high throughput printing of DNA submicron patterns on solid substrates (Wang et al., 2009). In this method, PMMA with submicron line or dot patterns (produced by NIL) was first chemically functionalized with poly(ethyleneimine) (PEI) to aminate the surface and then a DNA pattern was prepared on the surface. PEI modification leads to higher DNA immobilization and DNA hybridization efficiency due to a higher density of amine functional groups provided by PEI. Next, complementary target DNA hybridized to the PMMA surface was transferred to an aminated glass slide through contact printing. The transfer mechanism is due to electrostatic attraction (between DNA and amine groups) which can overcome hydrogen bonds between two hybridized DNA molecules. DNA transferred from the PMMA to the glass slide can still hybridize with DNA having a complementary sequence. This method provides a simple and high throughput means of preparing uniform DNA submicron patterns which are difficult to prepare using conventional solution-based methods. NIL can also benefit from combination with scanning probe lithography methods. The combination of NIL and scanning force lithography (SFL) was used for processing features with a definite sidewall angle. In this method, the imprinted profiles are successfully post-processed within local regions by SFL (Schultz et al., 2000). Cutting of lines as well as line removal were demonstrated. In addition, controlled slope could be prepared by adjusting the local gradient of the load force in the SFL process.

10.10 Applications Since NIL can be used to replicate patterns with feature sizes as smaller than 10 nm, NIL, and specifically step-and-flash imprint lithography (S-FIL), is a leading contender for IC fabrication of sub-50 nm features at the 22 nm and 18 nm nodes. S-FIL is well suited to semiconductor manufacturing because it is a relatively low-cost, room temperature, and low pressure process with resolution limited only by the size of the features on the template. Demonstrations of the high resolution of S-FIL through the development of imprint templates, imprint materials, and functional materials evidence the potential for this technology to become the primary candidate for device manufacturing (Costner et al., 2009). One can estimate that NIL is now in the stage of industrial pre-production with two most

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likely industrial application fields: (i) data storage (patterned media or discrete track recording hard disks) and (ii) optical components requiring fine-resolution patterning and 3D features (such as photonic crystals, sub-wavelength polarizers, high brightness LEDs, and backlighting for flat panel displays) where replication techniques will be able to replace standard methods of lithography (Schift, 2008). Apart from resolution requirements, this is simply because of lower costs as compared to the more established lithographic methods. The transition from the preproduction scale to industrial pre-production is supported by the availability of production machines such as the ‘Sindre’ high volume manufacturing by Obducat, Imprio® 250 and 1100 by MII, or EVG® 750 by EV Group (Schift, 2008). Concerning the first application field, usually the disk drive media for magnetic storage applications consist of thin magnetic films that are deposited on a substrate. The magnetic alloy films form nanometer-scale randomized grains that behave as the magnetic storage elements (bits). The increase of the storage capacity of these media is determined by the decrease of grain sizes of the magnetic films. However, with smaller grain sizes, the thermal stability of each bit is reduced. Patterned media can be used to overcome this effect by creation of a magnetic layer that is an ordered array of highly uniform islands of magnetic material surrounded by a nonmagnetic matrix. Each of these islands is capable of storing an individual bit (Costner et al., 2009). The patterning of magnetic media is a particular challenge with conventional lithographic techniques due to extremely small feature sizes that are beyond the resolution of current technology. On the other hand, S-FIL can be used as a potential solution. Due to its high scalability, NIL is being considered by several leading HDD manufacturers to extend their product roadmaps by enabling two possible recording methods: discrete track recording (DTR) and bit patterned media (BPM) (Glinsner et al.,2010). DTR increases recording density by forming a ‘groove’ between the tracks on the perpendicular magnetic recording (PMR) medium. The groove reduces signal interference between adjacent data tracks, allowing the pitch of the tracks to be shortened. In BPM, each magnetic bit is physically patterned onto the disk. In both DTR and BPM, the features produced by NIL are much smaller than those found in today’s most advanced semiconductor devices. High density templates on fused silica substrates for the UV imprint process have been recently fabricated. These templates were applied for imprints with a pitch of 24 nm (1.1 Tdots/ in.2) (Yang et al., 2008). Step and flash and jet and flash imprint lithography has been also applied to patterning hard disk substrates with both discrete tracks and bitpatterned designs (Schmid et al., 2009). CMOS image sensors and phase-change random access memory (PRAM) are two other areas of information processing and storage where the

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application of NIL is a reality. CMOS image sensors represent a multibillion-dollar market, with continued growth expected due to increasing demand for a multitude of consumer and industrial applications. NIL offers lower cost, high repeatability, and accuracy, which are particularly critical for ensuring optimal production of microlenses, which are an integral part of the CMOS image sensor. Today, the most established method of manufacturing wafer-level optics is sequential double-side microlens molding onto glass wafer substrates utilizing puddle dispense and UV-NIL (Glinsner et al., 2010) On the other hand, PRAM is one of the most promising non-volatile memories due to its ability to store digital data in the form of crystalline and amorphous phases of phase-change materials. In order to fabricate highly integrated PRAM devices, 70 nm-sized Ge2Sb2Te5 (GST225) phase-change media were patterned using NIL (Yang et al., 2010). In the second above-mentioned application field, NIL is widely used for the fabrication of photonic crystal and plasmonic crystal media, in which the alignment requirements are minimal and resolution and cost are the primary concerns (Resnick et al., 2007). Photonic crystals can be used to enhance the overall light efficiency and brightness of an LED (Kim et al., 2007; Glinsner et al., 2010). If the photonic crystals are fabricated with periods comparable to the optical wavelength within an LED, diffractive effects can be used to improve the extraction efficiency of the LED. Plasmonic crystals fabricated with precisely controlled arrays of sub-wavelength metal nanostructures provides a promising platform for sensing and imaging of surface binding events with micrometer spatial resolution over large areas. Soft NIL provides a robust, cost-effective method for producing highly uniform plasmonic crystals of this type with predictable optical properties (Yao et al., 2010). Apart from that, plasmonic nanohole and nanocones arrays fabricated by NIL are effective substrates for surface-enhanced Raman scattering (SERS) spectroscopy and imaging (Wu et al., 2010). NIL texturization is also a versatile technique for improving the light harvest efficiency and enhancing the power conversion efficiency of solar cells through a drastic reduction in reflectivity of the anti-reflection (AR) layers over a broad spectral range, which was demonstrated for both inorganic and organic solar cells (Zeng et al., 2009; Chen and Sun, 2010). In a more general aspect, NIL is an emerging technology for the fabrication of metamaterials. RT-NIL has been successfully applied to the fabrication of planar chiral photonic meta-materials (Chen et al., 2005). Among other emerging applications of NIL-based techniques one can mention the fabrication of nanoelectromechanical systems (NEMS) (for instance, interdigitated SiO2/metal double-finger cantilevers for optical switching elements), electrical filters, mass sensors, etc. (Luo et al., 2006), organic electronics as, for instance, polymeric distributed feedback lasers

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(Mele et al., 2006) and organic thin-film transistors (Auner et al., 2009, 2010), and a wide range of functional materials for sensors, microfluidic applications, tissue engineering, and drug delivery (Glinsner et al., 2010; Ofir et al., 2010). NIL was used for coating patterned Au substrates with protein arrays. These substrates then have a potential application as elements in the development of biosensors and other bioelectronic devices (Choi et al., 2009). The ability to precisely manipulate size, shape, and composition of nanoscale carriers is essential for controlling their in-vivo transport, biodistribution, and drug release mechanism. Shape-specific, ‘smart’ nanoparticles that deliver drugs or imaging agents to target tissues, primarily in response to disease-specific or physiological signals, could significantly improve therapeutic care of complex diseases. A nanoimprinting method for creating enzymatically triggered nanocarriers of precise sizes and shapes for drug and contrast agent delivery has been recently demonstrated (Glangchai et al., 2008). Particles as small as 50 nm were fabricated on silicon wafers and harvested directly into aqueous buffers using a biocompatible, one-step release technique. Successful encapsulation and precisely controlled enzyme-triggered release of antibodies and nucleic acids from these nanoparticles have been demonstrated, thus providing a potential means for disease-controlled delivery of biomolecules. Microfluidic devices for biomolecule separation using an array of well-defined nanostructures have been fabricated with NIL (Pepin et al., 2002). Two types of pattern replication of the same device configuration were considered, based on different material processing. In the first approach, a tri-layer NIL process was used to pattern a SiO2 substrate, on top of which a transparent elastomer cover plate was stuck. The second approach relies on direct imprinting of thermoplastic polymer pellets to form two bulk plastic plates later assembled together by thermal bonding. As a result, novel microfluidic devices combining deep and wide channels and a shallower nanostructure array were obtained. These realizations demonstrate not only that nanofluidic devices are achievable, but also that they can be manufactured for mass production via nanoimprint-based techniques.

10.11 Conclusions The steadily increasing scale of integration in microelectronics requires the development of alternatives to conventional lithography methods able to surpass photolithography in resolution, and, at the same time, to ensure high throughput for mass fabrication at a lower cost. NIL is actually the most promising approach to time and cost-effective fabrication of nanometerscale patterns, and is considered as a potential solution for the manufacturing of leading-edge semiconductor devices. The ITRS, which is established by leading experts in the semiconductor industry as a guide for directing

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research and development efforts, has identified the entry point for NIL technology in semiconductor fabrication at the 22-nanometer node, which several leading memory manufacturers are expected to reach in the 2011– 2012 timeframe. However, the replacement of optical lithography by NIL is not obvious. Among other NIL methods, the development of SFIL techniques has focused primarily on semiconductor nanofabrication. However, the successful implementation of SFIL still requires the development of new photocurable polymers and appropriate economics for the application. In order to achieve high throughput, imprint times of a few seconds will be needed, which is currently still difficult both for SFIL and for thermal imprint, because of limitations of the curing and air dissolution speed in SFIL, and the slow heating and cooling in thermal NIL. On the other hand, recent technology developments to extend existing optical lithography processes for semiconductor fabrication may delay the introduction of NIL to the 22-nanometer or smaller nodes, which will occur in later years. If one performs a cost of ownership analysis taking into account not only the cost of equipment, masks, but also the number of lithography steps, effect of machine occupancy, yield, the need of redundancy, and backup solutions, e.g., additional numbers of masks because of contamination or necessary reserves, one can come to the conclusion that NIL is not automatically less expensive than photolithography, nor is it the process of choice for any kind of application. NIL may be a solution for particular applications in areas where the cost of ownership of standard lithography is considered too high. For example, for the imprint of distributed feedback (DFB) gratings on a single wafer filled with hundreds of semiconductor lasers, a 5 min process time and even more would be acceptable (Schift, 2008). NIL still requires significant development, especially in the areas of overlay alignment accuracy and low defect density, before it can meet the extremely stringent manufacturing standards required for leading-edge semiconductor fabrication (Glinsner et al., 2010). In the long term, NIL can become an integral part of micro- and nanoprocessing with the concentration of efforts on real developments and innovative processes. At the same time, NIL is an unsurpassed technique for specific applications such as fabrication of photonic and plasmonic devices. Nanoimprinted plasmonic devices combine the exceptional performance of surface plasmon active substrates with rapid and consistent manufacturing techniques and inexpensive materials. These devices provide high analytical sensitivity over tunable wavelength ranges as well as versatile modes of operation. The integration of portable plasmonic devices with lab-on-a-chip microfluidic systems opens a promising route to achieve real-time label-free detection with sub-monolayer resolution (Yao et al., 2010).

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10.12 Acknowledgement This work was supported by the Supreme Council for Research and Technological Development of the Academy of Sciences of Moldova under the State Programme ‘Nanotechnologies and nanomaterials’.

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Unno N, Taniguchi J and Ishii Y (2007), ‘Sub-100-nm three-dimensional nanoimprint lithography’, Journal of Vacuum Science and Technology B, 25 (6), 2361–2364. Unno N, Taniguchi J, Ide S, Ishikawa S, Ootsuka Y, Yamabe K and Kanbara T (2009), ‘Three dimensional metal nanoimprint technique for electrode and electric probe’, Journal of Physics: Conference Series, 191, 012014. Wang Y, Goh SH, Bi X and Yang K-L (2009), ‘Replication of DNA submicron patterns by combining nanoimprint lithography and contact printing’, Journal of Colloid and Interface Science, 333 (1), 188–194. Weiss DN, Meyers ST and Keszlern DA (2010), ‘All-inorganic thermal nanoimprint process, Journal of Vacuum Science and Technology B, 28 (4), 823–828. Wu C-C, Hsu SLC and Liao W-C (2009), ‘A photo-polymerization resist for UV nanoimprint lithography’, Microelectronic Engineering, 86 (3), 325–329. Wu W, Hu M, Ou FS, Li Z and Williams RS (2010), ‘Cones fabricated by 3D nanoimprint lithography for highly sensitive surface enhanced Raman spectroscopy’, Nanotechnology, 21 (25), 255502. Xia Y and Whitesides GM (1998), ‘Soft lithography’, Angewandte Chemie International Edition, 37 (5), 550–575. Xia Y, Rogers JA, Paul KE and Whitesides GM (1999), ‘Unconventional methods for fabricating and patterning nanostructures’, Chemical Review, 99 (7), 1823–1848. Xia Q, Keimel C, Ge H, Yu Z, Wu W and Chou SY (2003), ‘Ultrafast patterning of nanostructures in polymers using laser assisted nanoimprint lithography’, Applied Physics Letters, 83 (21), 4417–4419. Yang K-Y, Hong S-H, Lee H and Choi J-W (2006), ‘Fabrication of nano-sized gold dot array using bi-layer nano imprint lithography’, Materials Science Forum, 510–511, 446–449. Yang XM, Xu Y, Seiler C, Wan L and Xiao S (2008), ‘Toward 1 Tdot/in.2 nanoimprint lithography for magnetic bit-patterned media: Opportunities and challenges’, Journal of Vacuum Science and Technology B, 26 (6), 2604–2610. Yang K-Y, Kim J-W, Hong S-H, Hwang J-Y and Lee H (2010), ‘Fabrication of nanoscale phase change materials using nanoimprint lithography and reactive ion etching process’, Thin Solid Films, 518 (20), 5662–5665. Yao J, Le A-P, Gray SK, Moore JS, Rogers JA and Nuzzo RG (2010), ‘Functional nanostructured plasmonic materials’, Advanced Materials, 22 (10), 1102–1110. Ye D, Wang P-I, Ye Z, Ou Y, Ghoshal R, Ghoshal R and Lu T-M (2010), ‘UV nanoimprint lithography of sub-100 nm nanostructures using a novel UV curable epoxy siloxane polymer’, Microelectronic Engineering, 87 (11), 2411–2415. Yokoo A (2003), ‘Nanoelectrode lithography and multiple patterning’, Journal of Vacuum Science and Technology B, 21 (6), 2966–2969. Zaidi SH and Brueck SRJ (1993), ‘Multiple-exposure interferometric lithography’, Journal of Vacuum Sciene and Technology B, 11 (3), 658–666. Zelsmann M, Toralla KP, De Girolamo J, Boutry D and Gourgon C (2008), ‘Comparison of monomer and polymer resists in thermal nanoimprint lithography’, Journal of Vacuum Science and Technology B, 26 (6), 2430–2433. Zeng W, Chong KSL, Low HY, Williams EL, Tam TL and Sellinger A (2009), ‘The use of nanoimprint lithography to improve efficiencies of bilayer organic solar cells based on P3HT and a small molecule acceptor’, Thin Solid Films, 517 (24), 6833–6836.

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11 Ultra-thin membranes for sensor applications I. TIGINYANU, V. URSAKI and V. POPA, Academy of Sciences of Moldova, Republic of Moldova

Abstract: Technological approaches for the fabrication of ultra-thin membranes for sensor applications are reviewed, with the main focus on graphene and two-dimensional (2D) sheets of layered compounds such as BN, MoS2, Bi2Te3, Bi2Se3. Highly conducting and transparent electrodes based on graphene are promising for use in flexible, stretchable, foldable electronics. The possibility of building multifunctional three-dimensional (3D) nanoarchitectures based on 2D graphene hybridized with one-dimensional (1D) semiconductor nanostructures is highlighted. The chapter also reviews the fabrication of ultra-thin GaN membranes of nanometer scale thickness by using the concept of surface charge lithography based on low-energy ion treatment of the sample surface with subsequent photoelectrochemical etching. Key words: layered materials, graphene, boron nitride, mechanical exfoliation, gallium nitride, membranes, surface charge lithography.

11.1

Introduction

Over the last years, there has been a significant amount of research exploring ultra-thin membranes of solids, including sheets of single-atomic-layer and few-atomic-layer-graphene, BN, MoS2, Bi2Te3, Bi2Se3 (Novoselov et al., 2004, 2005; Pacilé et al., 2008; Meyer et al., 2009; Hong et al., 2010; Mak et al., 2010; Splendiani et al., 2010; Teweldebrhan et al., 2010). Andre Geim and Konstantin Novoselov from the University of Manchester were awarded the 2010 Nobel Prize in Physics for their ‘groundbreaking experiments regarding the two-dimensional (2D) material graphene’. Note that graphite and all mentioned binary compounds represent layered materials characterized by strongly anisotropic chemical bonding where adjacent structural units are coupled by weak van der Waals interaction. It is this weak interaction that allows mechanical exfoliation of ultra-thin membranes, successfully realized in the first experiments by Geim and Novoselov just by using simple Scotch tape. Below we will describe the achievements in the fabrication of graphene as well as of sheets of binary compounds with layered structures such as BN, MoS2, Bi2Te3, Bi2Se3. Moreover, we will demonstrate the possibility of fabricating nanometer-thin membranes of wurtzite GaN possessing strong ionic–covalent chemical bonds which in our opinion may 330 © Woodhead Publishing Limited, 2011

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be considered as a breakthrough in the technology of manufacturing ultrathin membranes based on non-layered solid-state materials.

11.2

Graphene and related two-dimensional (2D) structures

In spite of theoretical predictions that a 2D lattice cannot survive at any finite temperature, researchers have succeeded in extracting single sheets of graphite. This unexpected stability (Novoselov et al., 2005), combined with the exotic band structure and other unusual physical properties of graphene (Geim and Novoselov, 2007), has led to a considerable amount of experimental research. Two methods have been used with considerable success for obtaining graphene: the first uses annealing of a SiC substrate to create a few-layer graphene surface (Berger et al., 2004) while the second uses micromechanical cleaving (Novoselov et al., 2005). With the second method, a fresh surface of a layered crystal was rubbed against another surface (virtually any solid surface is suitable), which left a variety of flakes attached to it. However, there are limiting aspects to each of these methods. In the case of micro-mechanical cleaving these include sample size and inadaptability to large-scale applications. In the case of annealed SiC, there are questions regarding sample quality and the method is relevant only to graphene. The properties of suspended graphene are attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal, and other characteristics of this one-atom-thick material. A simple, inexpensive and fast method for producing larger graphene samples, and 2D samples of layered materials in general, was proposed (Shukla et al., 2009). This method is inspired from a technique used for bonding Si to a Pyrex substrate, known as anodic bonding. By applying a potential difference of the order of a kV to a heated Pyrex/Si interface, very intimate contact is obtained between the substrate and silicon which translates into the formation of chemical bonds at the interface. In the case of the Si/Pyrex interface the bond is permanent and irreversible. For the preparation of graphene, voltage from 1.2–1.7 kV was applied with the anode on the graphite sample and the cathode contacting the backside of the Pyrex substrate. The glass substrate with the graphite mounted on it was heated. With the temperature stabilized, the potential difference was applied and a current was immediately detected, signalling the migration of the sodium ions inside the glass substrate. As the space charge layer is formed and begins to counteract the applied voltage, the current decreases exponentially. Bonding within a few minutes both with

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500 mm thick Pyrex-7740 substrates as well as ordinary 120 mm thick laboratory borosilicate cover-slips was achieved. After bonding was achieved, the bulk graphite sample was cleaved off, leaving several bonded areas on the glass surface. These were then peeled off using adhesive tape to leave many transparent areas with one-layer, or few-layer graphene portions. This simple, inexpensive and fast method leads to the preparation of large-area graphene and single or few-layer films of layered materials in general. Millimeter-sized few-layer graphene samples have been prepared and I–V characteristics in a field-effect transistor (FET) have been measured. Another highly reliable approach for making graphene membranes of a macroscopic size (up to 100 μm in diameter) was proposed by Booth et al. (2008). The procedure does not involve any aggressive etchants that can lead to the ‘oxidation’ of graphene and/or its irreversible contamination, which makes the technique suitable for incorporation into complex microfabrication pathways. Figure 11.1 explains the fabrication steps involved. Graphene crystals are first prepared by standard micromechanical cleavage techniques. Sufficiently large flakes produced in this way are widely distributed over a substrate (occurring with a typical number density of <1 per cm2). Fortunately, one-atom-thick crystals can be identified on surfaces covered with thin dielectric films because of a color shift induced by graphene, which allows crystals to be found rapidly with a trained eye and an optical microscope. Si wafers were used that, in contrast to the standard approach, are not oxidized but instead covered with a 90 nm thick film of polymethyl methacrylate (PMMA) (referred to as a base layer in Fig. 11.1a). The optical properties of PMMA are close to those of SiO2, and the visible contrast of graphene is optimal at this particular thickness. The PMMA film also serves later as a sacrificial layer during the final lift-off. Once a suitable graphene crystal is identified in an optical microscope, photolithography was employed to produce a chosen pattern (a transmission-electron microscopy–TEM–grid in this case) on top of graphene (a double-layer resist consisting of 200 nm polymethyl glutarimide (PMGI) and 200 nm S1805; Fig. 11.1a,b). A 100 nm Au film with a 5 nm Cr adhesion layer was thermally evaporated after developing the resist (Fig. 11.1c). Liftoff of the metal film is performed in a 2.45 wt % tetramethylammonium hydroxide (TMAH) solution (Fig. 11.1d). The next step involves another round of photolithography (Fig. 11.1e) in which the graphene crystal is remasked with the same photoresist. The mask serves here to protect graphene during electrodeposition, when a thick copper film is electrochemically grown on top of the Au film, repeating the designed pattern (Fig. 11.1f). Finally, acetone is used to strip the remaining resist, releasing the copper TEM grid with the attached graphene membrane (Fig. 11.1g). A copper thickness of 10–15 μm was found to be sufficiently robust for reliable handling of the samples.

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Graphene flake Double layer resist Base layer Si wafer (e)

(a)

Cu

(b) Au

(f)

(c)

(d)

(g)

11.1 Microfabrication steps used in the production of graphene membranes. (Reprinted from Booth TJ, Blake P, Nair RR, Jiang D, Hill EW, Bangert U, Bleloch A, Gass, Novoselov KS, Katsnelson MI and Geim AK (2008), ‘Macroscopic graphene membranes and their extraordinary stiffness’, Nano Letters, 8 (8), 2442–2446. Copyright (2008) with permission from American Chemical Society)

It was found that long graphene beams supported by only one side do not scroll or fold, and demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight. This technology for the preparation of suspended graphene opens many avenues for implementation in various micromechanical systems and electron microscopy. Several methods have been explored to date to obtain graphene in solution phase by means of chemical routes. High quality graphene nanoribbons (GNR) have been obtained by sonicating thermally exfoliated graphite in a 1,2-dichloroethane (DCE) solution of poly(m phenylenevinyleneco-2,5dioctoxy-p-phenylenevinylene) (PmPV) (Li et al., 2008a). However, the yield was low and most of the ribbons had two or more layers. It was recently reported that the exfoliation–reintercalation–expansion of graphite can produce high quality single-layer grapheme (SLG) sheets stably suspended in organic solvents (Li et al., 2008b). Large amounts of graphene sheets in organic solvents are made into large transparent

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conducting films by Langmuir–Blodgett assembly in a layer-by-layer manner. To make high quality graphene sheets, commercial expandable graphite was exfoliated in the first step by brief (60 s) heating to 1000 °C in forming gas. Next, the exfoliated graphite was ground, reintercalated with oleum, and inserted with tetrabutylammonium hydroxide (TBA) solution in water in the second step. Then the TBA inserted oleum-intercalated graphite was sonicated in the third step in a dimethylformamide (DMF) solution to form a homogeneous suspension. Centrifugation was used in the fourth step to remove large pieces of material from the supernatant. This method resulted in large amounts of graphene sheets suspended in DMF and can be transferable to other solvents including water and organic solvents. High quality graphene sheets (almost pristine graphene) in large quantities were prepared by a two-step method for obtaining a homogenous colloidal suspension of single or few-layer graphene (FLG) sheets up to 0.15 mg ml−1 in N,N-dimethylformamide solution (Singh et al., 2010). The two-step approach includes: (i) high temperature (2000 °C) heat treatment of commercially available pyrolytic graphite powders in a vacuum for 3 h (this heat treatment may reduce contamination) followed by (ii) sonication for 2 h by dissolving the heat-treated graphite in DMF using a probe-tip sonicator. It was found that the pyrolytic graphite is composed of a periodical stack of 2D graphene sheets (layers) along the c-axis. Each of these layers is weakly bonded to its neighboring layers by interlayer interaction forces; the graphene layers can easily slide against each other and peel off easily. As a result of the heat treatment of pyrolytic graphite at high temperature under vacuum, the interactions between graphene layers may soften, thus making their liquid-phase exfoliation with DMF easier. By filtering in the frequency domain using a Fourier transform, the graphene lattice of each sheet was reconstructed and the relative rotation between consecutive graphene layers was determined. Figure 11.2(a) shows a bright-field TEM image of graphene crystallites attached to a TEM grid. The suspended graphene membranes consist of SLG or FLG sheets. The two-layered graphene is clearly visible and it is marked by the lower right square in Fig. 11.2a. Folded regions in the FLG sheets, which give rise to Moire patterns from rotational stacking faults are also observed (see the lower left box in Fig. 11.2b). Figure 11.2b depicts a highly magnified image of the region indicated by the left square in Fig. 11.2a. Figure 11.2b describes single-layer, two-layer, and four-layer graphene sheets, respectively. A 2D fast Fourier transform (FFT) was performed in the region indicated by the upper right box in Fig. 11.2b. The FFT of a single hexagonal graphene network produces six spots of 0.21 nm spacing which corresponds to a single layer. Figure 11.2c exhibits a high resolution TEM (HRTEM) image of SLG, which was acquired from the region indicated with the upward pointing dotted arrow. Figure 11.2d shows

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11.2 HRTEM images of a freely suspended graphene membrane. (a) Bright-field TEM image of a suspended graphene membrane. (b) Magnified view of the region denoted by the left box in (a); the inset shows 2D FFT performed in the region indicated with the upper box. (c) HRTEM image of single-layer graphene acquired from the region indicated with the up pointing dotted arrow in (b). (d) Reconstructed image after filtering in the frequency domain to remove unwanted noise, for clarity. The inset shows the hexagonal graphene network. (Reprinted from Singh MK, Titus E, Goncalves G, Marques PAAP, Bdikin I, Kholkinb AL and Gracio JJA (2010), ‘Atomicscale observation of rotational misorientation in suspended few-layer graphene sheets’, Nanoscale, 2 (5), 700–708. Copyright (2010) with permission from Royal Society of Chemistry)

the image reconstructed by filtering in the frequency domain to remove unwanted noise. The hexagonal graphene network is clearly resolved in the inset of Fig. 11.2d, showing the magnification of the small region indicated with the box. The C–C bond length is measured to be 1.4 Å ± 0.2 Å and the measured lattice constant is 2.5 Å ± 0.2 Å, confirming that this is a single layer. A direct chemical synthesis of carbon nanosheets in gram-scale quantities has been reported in a bottom-up approach based on the common laboratory reagents ethanol and sodium, which are reacted to give an intermediate

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solid that is then pyrolized, yielding a fused array of graphene sheets that are dispersed by mild sonication (Choucair et al., 2009). The ability to produce bulk graphene samples from non-graphitic precursors with a scalable, low cost approach is expected to accelerate the applications of graphene. Graphene samples with areas of several square centimetres and excellent electrical and optical properties have also been fabricated using chemical vapor deposition (CVD) on nickel (Kim et al., 2009) and copper (Li et al., 2009) substrates. Different methods of patterning the graphene films and transferring them to arbitrary substrates have also been presented, and the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics has been demonstrated. The rollto-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by CVD onto flexible copper substrates have been reported (Bae et al., 2010). The layer-by-layer stacking was used to fabricate a doped four-layer film with the sheet resistance at values as low as ∼30 Ω per square at ∼90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain. Apart from graphene, reduced graphene oxides (RG-Os) have attracted considerable interest, due to their potential applications in electronic and optoelectronic devices and circuits. Graphene oxide (GO) is a chemically modified graphene containing oxygen functional groups such as epoxides, alcohols, and carboxylic acids, and chemical analysis shows the carbon to oxygen ratio to be approximately three to one. Recently, GO has received a great deal of attention because it readily exfoliates as single sheets in water, it is straightforward to produce continuous films and from these solutions. This affords GO a distinct advantage over fullerenes which are typically deposited as films by use of high temperatures and vapor transport and should allow the use of plastic substrates or other temperature-sensitive processes. It was shown that RG-Os readily allow the detection of chemical agents in the parts-per-billion range with significantly reduced noise levels over carbon nanotube-based sensors (Robinson and Perkins, 2008). These graphene oxide networks are tunably reduced towards graphene by varying the exposure time to a hydrazine hydrate vapor. The level of reduction affects both the sensitivity and the level of 1/f noise. It was suggested that oxidized graphene sheets stacked atop one another like the decks of a multilevel parking lot, connected by molecules that link the layers to one another and maintain space between them, can accumulate hydrogen in great quantities. It was found that a graphene-oxide framework (GOF) that consists of layers of GO connected by benzene-1,4-diboronic acid (B14DBA) pillars can hold at least a hundred times more hydrogen

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molecules than ordinary graphene oxide does (Burress et al., 2010). The easy synthesis, low cost, and non-toxicity of graphene make this material a promising candidate for gas storage applications. The development of strong and cost-efficient multifunctional graphenefilled polymer composites is also in progress. A key challenge in the fabrication of nanoplatelet-filled polymer composites is the ability to realize the nanometer-level dispersion and the planar orientation of nanosheets in polymer matrices. Ultra-thin multilayer (PVA/GO)n films were successfully fabricated by bottom-up layer-by-layer (LBL) assembly of poly(vinyl alcohol) (PVA) and exfoliated GO, in which exfoliated GO nanosheets were used as the building blocks (Zhao et al., 2010). A significant enhancement of mechanical properties has been achieved. This may be attributed to the well-defined layered architecture with high degree of planar orientation and nanolevel assemblies of GO nanosheets in the polymer matrices. Among other graphene-based materials, of especial interest are 2D graphene hybridized with one-dimensional (1D) semiconductor nanostructures which enable the construction of three-dimensional (3D) architectures and the imposition of multifunctionalities. The synthesis of 1D–2D hybrid architectures (HAs) composed of regular arrays of ZnO nanorods formed on graphene layers was demonstrated (Lee et al., 2009). The 1D–2D HAs exhibited outstanding electrical conductivity, optical transparency, and mechanical flexibility, comparable to those of graphene. In addition, new optical functions inherited from the ZnO nanorods were introduced to the 1D–2D HAs, which, combined with the excellent electrical and mechanical properties, suggests a wide spectrum of applications ranging from transparent conducting electrodes to active components in wearable and flexible electronic, photonic, and photovoltaic systems. The key steps in the overall fabrication process include (i) large-area synthesis of graphene, (ii) transfer of graphene to an arbitrary substrate, and (iii) low temperature selective growth of ZnO nanorods on graphene. The optical microscope (OM) image of an as-fabricated ZnO–G HA on a glass substrate (Fig. 11.3a) revealed that the regular array of ZnO crystals (dark spots) was formed over a large area (∼1 cm × 1 cm) as a continuous graphene film. The scanning electron microscope (SEM) image of the sample shows that the dark spots corresponded to a high density of ZnO nanorods with a mean diameter and length of ∼100 nm and ∼7 μm, respectively, grown via holes in the graphene layer (Fig. 11.3b). Specifically, the ZnO nanorods were close-packed and oriented in the radial direction, which led to the formation of flower-like ZnO nanorod bundles with a typical diameter of ∼15 μm. Since the morphologies and crystallographic orientations of ZnO nanorods are generally affected by the ZnO seeding layers, the nanorod growth was also investigated using a ZnO epilayer grown on a sapphire substrate. As shown in Fig. 11.3c, hexagonal-faceted

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(a)

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11.3 Morphological and structural analyses of the ZnO–G HAs. (a) OM and (b) SEM images of a ZnO–G HA on glass coated with the textured ZnO layer. (c) SEM image of a ZnO–G HA grown using the ZnO epilayer as a seeding layer. (d) Representative Raman spectra of the graphene transferred to a bare glass substrate (G/glass) and the ZnO–G HA on a glass substrate coated with a ZnO layer recorded with the excitation wavelength of 514 nm. (Reprinted from Lee JM, Pyun YB, Yi J, Choung JW and Park WI (2009), ‘ZnO nanorod–graphene hybrid architectures for multifunctional conductors’, Journal of Physical Chemistry C, 113 (44), 19134–19138. Copyright (2009) with permission from American Chemical Society)

ZnO rods with diameters in the range of 5–10 μm grew vertically at the center of the graphene holes. The high quality of the graphene layers in the ZnO–G HAs was confirmed by Raman measurements. Figure 11.3d compares the Raman spectra of the graphene transferred to the bare glass substrate (G/glass) and the ZnO–G HA fabricated on the glass substrate coated with a textured ZnO seeding layer. A cyclic bending test for the ZnO–G HAs on plastic substrates showed that reiterations between spotty and streaky diffraction patterns were repeated up to tens of cycles, which reflects the excellent mechanical flexibility and structural stability of the HAs. The unique electrical, optical, and mechanical properties of the 1D–2D HAs demonstrate the potential application as multifunctional conducting layers in electronic and optoelectronic systems.

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Since graphene was exfoliated from graphite in 2004, the 2D monoatomic sheet has attracted increased interest for both fundamental studies and applications in high-speed electronic devices, sensors, memory, and spintronic devices among others (Geim and Novoselov, 2007). Graphene has high mobility and optical transparency, in addition to flexibility, robustness, and environmental stability. The combination of its unique optical and electronic properties can be fully exploited in photonics and optoelectronics. The rise of graphene in photonics and optoelectronics is shown by many recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors, and ultrafast lasers (Bonaccorso et al., 2010). Graphene can fulfil multiple functions in photovoltaic devices as transparent conductor window, photoactive material, channel for charge transport, and catalyst. Graphene transparent conducting films (GTCFs) can be used as window electrodes in inorganic (Fig. 11.4a), organic (Fig. 11.4b), and dye-sensitized solar cell devices (Fig. 11.4c). Graphene has a work function of 4.5 eV, similar to indium tin oxide (ITO). This, combined with its promise as a flexible and cheap TCF, makes it an ideal candidate for organic lightemitting diode (OLED) anodes (Fig. 11.4d), while also eliminating the issues related to indium diffusion into the active OLED layers, which is

(a) Light

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Output

11.4 Graphene-based optoelectronics. (a–c) Schematics of inorganic (a), organic (b) and dye-sensitized (c) solar cells. I− and I−3 are iodide and tri-iodide, respectively. The I− and I−3 ions transfer electrons to the oxidized dye molecules, thus completing the internal electrochemical circuit between the photoanode and the counter-electrode. (d,e) Schematics of an organic LED (d) and a photodetector (e). The cylinder in (d) represents an applied voltage. (Reprinted from Bonaccorso F, Sun Z, Hasan T and Ferrari AC (2010), ‘Graphene photonics and optoelectronics’, Nature Photonics, 4 (9), 611–622. Copyright (2009) with permission from Nature Publishing Group)

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inherent to ITO-containing devices. GTCFs anodes enable an out-coupling efficiency comparable to ITO. Since graphene absorbs from the ultraviolet to terahertz range, graphene-based photodetectors (GPDs; see Fig. 11.4e) could work over a much broader wavelength range as compared to photodetectors based on IV and III–V semiconductors which suffer from the ‘long-wavelength limit’, as these become transparent when the incident energy is smaller than the bandgap. GPDs can be ultrafast, since the response time is ruled by the carrier mobility, while graphene has huge mobilities. GTCFs can satisfy the requirements for resistive touch screens in terms of optical transparency and electrical resistance. A graphene-based touch panel display has been produced by screen-printing a CVD-grown sample (Bae et al., 2010). The ultrafast carrier dynamics, combined with large absorption and Pauli blocking, makes graphene an ideal ultra-broadband, fast saturable absorber for ultra-fast lasers (Bonaccorso et al., 2010). Unlike semiconductor saturable absorber mirrors and single-walled carbon nanotubes (SWNTs), there is no need for bandgap engineering or chirality/diameter control to assure broadband tenability. So far, graphene–polymer composites, CVD-grown films, functionalized graphene, and RG-O flakes have been used for ultrafast lasers. Graphene can be also used for optical limiters, optical frequency converters, and terahertz devices. In graphene-based optical limiters the absorbed light energy converts into heat, creating bubbles and microplasmas, which results in reduced transmission. Graphene dispersions can be used as wideband optical limiters covering visible and near-infrared. The possibility of tuning the electronic and optical properties by external means (for example, through electric or magnetic fields, or using an optical pump) makes SLG and FLG suitable for infrared and terahertz radiation manipulation. The possible devices include modulators, filters, switches, beamsplitters, and polarizers. Graphene is also an exceptional material for chemical sensors with a level of sensitivity such that individual atoms or molecules can be resolved. It was shown that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene’s surface (Schedin et al., 2007). The adsorbed molecules change the local carrier concentration in graphene one electron at a time, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required. Owing to its exceptional carrier mobility and one-atomic thickness, graphene field effect transistors (Gra-FETs) have been proposed to hold great

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11.5 Graphene FETs in the electrolyte solution. (a) Schematic representation of the experimental setup where a single-layer graphene is supported in solution by Cr/Au contacts to bridge a trench in the oxide. (b) In situ etching of SiO2 underneath graphene. The conductance of graphene starts to drop gradually after buffered HF was added to the poly(dimethylsilokane) (PDMS) chamber. Single layer devices usually stabilize within 50 to 100 s, indicating the complete suspension of graphene in solution. Arrows indicate the time when solution was switched in the PDMS chamber. The inset shows a scanning electron microscope (SEM) image taken of a suspended graphene device after solution measurements. Scale bar is 0.5 μm. (Reprinted from Cheng Z, Li Q, Li Z, Zhou Q and Fang Y (2010), ‘Suspended graphene sensors with improved signal and reduced noise’, Nano Letters, 10 (5), 1864–1868. Copyright (2010) with permission from American Chemical Society)

potentials for sensitive and label-free detection of chemical/biological species. As an example, suspended devices were demonstrated (see Fig. 11.5a) as real-time and sensitive pH sensors (Cheng et al., 2010), and complementary detection with either holes or electrons as charge carriers in the same graphene device was achieved (Fig. 11.5b), opening up new opportunities for suspended graphene as flexible candidates for bioelectronics. Most important of all applications, graphene may form the basis of a new breed of computer chips, smaller and faster than those based on silicon. With silicon computing expected to reach its limits in 15–20 years time, there is intense interest in graphene-based replacements.

11.3

Nanometer-thick membranes of layered semiconductor compounds

Apart from producing graphene, the possibility of extracting and isolating 2D ordered crystals composed of elements other than carbon is a new

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frontier of physics. Novoselov et al. (2005) suggested the possibility of extracting single sheets of other layered materials by the simple technique of micromechanical cleavage. They performed microscopy investigations of dichalcogenides NbSe2 and MoS2, layered superconductors Bi2Sr2CaCu2Ox, and commented on the possibility of obtaining similar results on layered boron nitride compounds. The hexagonal boron nitride (h-BN) phase (sometimes called white graphite) is isostructural to graphite except for the different stacking sequences of the atomic planes: well-crystallized graphite displays the Bernal (AB) stacking sequence, while hexagonal boron nitride is stacked with boron on top of nitrogen and vice versa (AAA⋅⋅⋅ stacking) (Pacilé et al., 2008). Although the BN and carbon-based materials have very similar crystal structure, their electronic properties are very different, as h-BN is an insulator with a direct energy gap of about 5.9 eV, while graphite is a semimetal. Single-layer boron nitride is the thinnest ionic material that can exist in two dimensions. Similar to graphene, this material offers a unique model system to study the stability and dynamics of defects, edges, and vacancies, and their interactions with the adatoms in ionic crystals. The first isolation of thin sheets of h-BN with the micromechanical cleavage technique was reported by Pacilé et al. (2008). The experimental study established their crystallinity and continuity over several microns. Both thin (few atomic-layer-thick) BN sheets deposited on oxide surfaces and similarly thin BN membranes freely suspended across circular apertures have been produced. Flakes of less than 10 layers have been afterwards prepared by mechanical cleavage and they were thinned down to single layers in a high energy electron beam (Meyer et al., 2009). The almost identical lattice constant with graphite or graphene suggests a joint use in composite devices. By analogy to the importance of the silicon dioxide insulator in the siliconbased microelectronics, it was suggested that BN layers might serve a similar role in graphene-based devices. Another method to prepare monolayer and multilayer suspended sheets of hexagonal boron nitride was presented using a combination of mechanical exfoliation and reactive ion etching (Alem et al., 2009). It was suggested that this exfoliation technique can be applied to similar materials to create atomically thin two-dimensional sheets at large size scales. Similarly to graphene, large area synthesis of h-BN films consisting of two to five atomic layers has been demonstrated using CVD (Song et al., 2010). One of the most feasible methods to control the semiconducting properties of graphene is by doping, which is a process intentionally used to tailor the electrical properties of intrinsic semiconductors. Experimental and theoretical studies on graphene doping demonstrated the possibility of making p-type and n-type semiconducting graphene by substituting C atoms with B and N atoms, respectively. More interestingly, B, C, and N can

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be atomically mixed together to form various semiconducting hexagonal layered structures with varying stoichiometry. Atomic sheets containing hybridized bonds involving elements B, N, and C over wide compositional ranges could result in new materials with properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties, and applications. The synthesis and characterization of largearea atomic layers of h-BNC material, consisting of hybridized, randomly distributed domains of h-BN and C phases with compositions ranging from pure BN to pure graphene has been reported (Ci et al., 2010). These studies revealed that their structural features and bandgap are distinct from those of graphene, doped graphene, and h-BN. This new form of hybrid h-BNC material enables the development of bandgap-engineered applications in electronics and optics and properties that are distinct from those of graphene and h-BN. Transition metal chalcogenides are another class of materials which allow the preparation of monolayer sheets. MoS2 is a prototypical transition metal chalcogenide material composed of covalently bonded S–Mo–S sheets that are bound by weak van der Waals forces. Since MoS2 is a layered d-electron material, it is expected to exhibit quantum confinement effects drastically different from those found in sp-bonded semiconductor nanostructures. The photoluminescence investigations demonstrated that MoS2, an indirect bandgap material in its bulk form, becomes a direct bandgap semiconductor when thinned to a monolayer (Splendiani et al., 2010). A systematic study of the evolution of the optical properties and electronic structure of ultra-thin MoS2 crystals as a function of layer number N = 1; 2; 3; . . . ; 6 allowed one to trace the evolution of both the indirect and direct bandgaps of the material as a function of layer thickness N (Mak et al., 2010). It was found that the freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material. Monolayer MoS2 constitutes the first atomically thin material that is an effective emitter of light. Its strong photoluminescence suggests its use for photostable markers and sensors that can be adapted to probe nanoscale dimensions. Such behavior, arising from d-orbital-related interactions in MoS2, may also arise in other layered transition metal dichalcogenides. It points out a new direction for controlling electronic structure in nanoscale materials by exploiting rich d-electron physics. Such capability can lead to engineering novel optical behaviors not found in sp-bonded materials and holds promise for new nanophotonic applications (Splendiani et al., 2010). Bi2Se3, Bi2Te3, and Sb2Te3 topological insulator are also layered materials in which two Bi or Sb atomic layers and three Se or Te atomic sheets are covalently bonded to form one quintuple layer (QL, ∼1 nm thick), where adjacent QLs are coupled by relatively weak van der Waals interaction

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allowing exfoliation down to a few QLs. The particles in topological insulators coated with thin ferromagnetic layers were proposed for possible applications in the magnetic memory where write and read operations are achieved by purely electric means. Apart from that, bismuth telluride is a vital component for the thermoelectric industry. A drastic improvement in the thermoelectric figure of merit can be achieved in low dimensional structures by electrons (holes) confinement or by reducing the thermal conductivity via spatial confinement of acoustic phonons. In order to employ the full strength of the low dimensional confinement effects for improving thermoelectric figure of merit either via the electron band-structure or via phonon engineering one needs to produce quasi-2D structures with a fewatomic-layer thickness and high quality interfaces. A ‘graphene-inspired’ mechanical exfoliation procedure from bulk Bi2Te3 has been proposed, which provides individual large-area quasi-2D crystals with many interesting properties (Teweldebrhan et al., 2010). Controlled mechanical exfoliation of Bi2Se3 nanoribbons (>50 QLs) down to a single QL was performed by an atomic force microscope (AFM) tip (Hong et al., 2010). AFM has been demonstrated to be a very effective and powerful technique to generate ultra-thin layered structures. Bi2Te3 and Bi2Se3 nanoplates with thickness down to 3 nm (3 QLS) have also been produced via catalyst-free vapor–solid (VS) growth mechanism (Kong et al., 2010).

11.4

Ultra-thin membranes of gallium nitride

Since year 2000, many international research groups have been focused on developing and optimizing gallium nitride nanostructuring. GaN nanorods (Park et al., 2006; Li et al., 2010a), nanowires (Hersee et al., 2006; Kuykendall et al., 2004; Tiginyanu et al., 2003), nanotubes (Goldberger et al., 2003), nanopyramids (Ursaki et al., 2007), etc. have been fabricated by various methods including molecular beam epitaxy, metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy, photoelectrochemical etching (PEC) techniques, etc. Recently Mei et al. (2009) demonstrated that ultra-thin AlN/GaN nanomembranes grown on (111)-oriented Si can be released by the selective etching of the silicon substrate, and self-assembled into various geometries such as tubes, spirals, and curved sheets. Nanopores with sizes ranging from a few to several tens of nanometers were produced in 20–35 nm thick nanomembranes due to island growth of AlN on Si. Müller et al. (2009) fabricated 500 nm thick GaN membranes as mechanically strong as to support film bulk acoustic resonator structures for frequencies up to 6.3 GHz with a very high quality factor. In the process of membrane fabrication, the Si substrate was selectively etched against GaN in a SF6 plasma. The authors attributed the good mechanical behavior of the thin membranes to the low stress in the GaN layer.

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To fabricate GaN membranes with nanometer-scale thickness we used the concept of surface charge lithography (SCL). To our knowledge, this is the first successful fabrication of nanometer-thin membrane on GaN. The experiments were realized using unintentionally doped wurtzite n-GaN layers grown by low-pressure MOCVD on (0001) c-plane sapphire substrates. The concentration of free electrons was of the order of 1017 cm−3, while the density of threading dislocations was in the range of 109–1010 cm−2. Selected areas of the GaN epilayers were subjected to treatment by Ar+ ions with energies of 0.4 or 1 keV, the fluence being equal to 1011 cm−2 for 0.4 keV ions, and to 1012 cm−2 for 1 keV ions. According to the concept of SCL, the treatment of the sample surface by low energy ions creates deep acceptors which trap electrons thus forming a shield of negative charge that protects the material against PEC dissolution. Note that Monte Carlo SRIM-2008 simulations (Ziegler et al., 2008) predict the main projected range of the 0.4 keV Ar+ ions in the GaN matrix to be 1.1 nm with a longitudinal straggling of 0.8 nm, while for 1 keV Ar+ ions the projected range is 1.7 nm with a longitudinal straggling of 1.2 nm. PEC etching was carried out in a stirred 0.1 mol aqueous solution of KOH for 5–10 min under in-situ ultraviolet (UV) illumination provided by focusing the radiation of a 350 W Hg lamp to a spot of about 5 mm in diameter on the sample surface. In the particular case of pre-treatment by 1 keV Ar+ ions, the duration of the etching process was up to 50 min. No bias was applied to the sample during etching. The morphology of samples was studied using a VEGA TESCAN TS 5130MM SEM and a NANOSTATION AFM. A JEOL 7001F field emission SEM equipped with a Gatan XiCLone cathodoluminescence (CL) microanalysis system was used for comparative morphological and CL characterization. The monochromatic CL images were collected using a Peltier cooled Hamamatsu R943-02 high sensitivity photomultiplier tube. The CL spectra have been excited with a 10 keV, 3.5 nA electron beam from ∼350 nm diameter areas of typical regions of the specimen. The spectra have been collected with a Pixis 100 CCD camera with 300 l/mm grating blazed at 500 nm, and corrected for instrument response. Figure 11.6a shows the morphology of a GaN sample subjected to PEC etching under weak stirring of the electrolyte. Note that, prior to the PEC etching, the left part of the sample shown in the image was pre-treated by 0.4 eV Ar+ ions. According to the concept of SCL, this led to the formation of deep acceptors trapping electrons with the excess negative charge then shielding the material against PEC dissolution. A nanometer-thick membrane was formed in the Ar-ion treated areas, while the material underneath the membrane was etched with the exception of whiskers representing threading dislocations. The extra-thin membrane has been found to be transparent to both electrons and UV radiation, with good transparency to UV radiation providing conditions for PEC etching to occur underneath

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the membrane. As a result, after etching the architecture of GaN consists of a top ion-treated area in the form of nm-thin membrane and threading dislocations that survive due to their negative charge (see the left part of Fig. 11.6a). Exploring the morphology of many samples subjected to PEC etching allowed us to observe perforation of the suspended membrane by tiny holes (see e.g. Fig. 11.6b) which seem to be responsible for the membrane permeability to electrolyte species during etching. We found that the topography of the membrane depends upon the electrolyte stirring intensity during PEC etching. Under relatively intense stirring conditions, we evidenced the formation of undulating membranes resembling waves (see left part of Fig. 11.6c and Fig. 11.6d). In this case, the

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11.6 SEM images showing top oblique views taken from 0.4 keV Ar-ion pre-treated and PEC-etched GaN samples: weak stirring of the electrolyte (a,b), and intense stirring of the electrolyte (c,d). (Reprinted from Tiginyanu IM, Popa V and Stevens-Kalceff MA (2011), ‘Ultra-thin GaN membranes fabricated by using surface charge lithography’, ECS Transactions, 35 (6), 13–19. Copyright (2011) with permission from The Electrochemical Society)

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membranes exhibit ruptures although the percolation of the material is often preserved. Note that even under relatively intense stirring it is possible to identify small areas where the membranes are continuous. This was demonstrated by atomic force microscopy (see the AFM topography image presented in Fig. 11.7). The analysis of many images typified by those in Figs 11.6c, 11.6d and 11.7 shows that the ‘wave’ crests are determined by networks of dislocations which in many cases exhibit well defined rows. Pre-treatment of GaN layers with 1 keV energy Ar+ ions at the dose of 1012 cm−2 led to the formation of membranes possessing higher mechanical strength. Since in this case the membranes demonstrated no permeability to electrolyte species, Ar+ ion pre-treatment was realized through windows in the form of relatively narrow strips allowing the electrolyte to subsequently penetrate under the emerging membrane from two lateral sides (Fig. 11.8a). Etching proceeds until the two etching fronts meet in the middle of the strip. We found that the detached pieces of the membranes can easily curl into rolls (Fig. 11.8b). An interesting feature, probably related to long-time etching in spatially confined conditions, consists of selective etching of whiskers located directly under the membrane (see upper part of the image presented in Fig. 11.8c). Often, in the central part of the strips, narrow areas of non-etched whiskers can be seen after removal of the membranes (Fig. 11.8d): As a result of the selective etching of whiskers in the two periphery regions of the strip, the respective periphery membrane

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11.8 SEM images showing top oblique views taken from 1.0 keV Ar-ion pre-treated and PEC-etched GaN samples (see text for details). (Reprinted from Tiginyanu IM, Popa V and Stevens-Kalceff MA (2011), ‘Ultra-thin GaN membranes fabricated by using surface charge lithography’, ECS Transactions, 35 (6), 13–19. Copyright (2011) with permission from The Electrochemical Society)

areas stick to the bottom surface thus stopping the penetration of the electrolyte under the membrane. Note that it is this phenomenon that does not allow fabrication of large-area non-permeable membranes. This is unambiguously demonstrated by the right-bottom corner of the image presented in Fig. 11.8a where it can be seen that the membrane emerging due to undercut etching sticks to the bottom surface, thus ceasing its own extension. Surprisingly, the ultra-thin membranes, when they are connected via electrical path to an earth electrode, appear relatively dark in the SEM images in contrast to the charging-effect-related brightness of the surrounding whiskers (see e.g. Fig. 11.6a, 11.6c, 11.8a, 11.8c). This is indicative of the good electrical conductivity of the membranes, despite their nanometer-scale thickness. According to recent theoretical calculations (Li et al., 2010b), GaN sheets and nanoribbons are expected to be ferromagnetic with defect-

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induced half-metallic configuration which is of particular interest for spintronic device applications. To realize a comparative study of the luminescence of the top membrane and threading dislocations, we explored the spatial distribution of micro-CL (μ-CL) within a selected region (Tiginyanu et al., 2011) illustrated by the SEM image in Fig. 11.9a. Monochromatic μ-CL images collected at wavelengths (energies) of 365 nm (3.4 eV) and 550 nm (2.25 eV) give insight into the CL emission associated with features of the GaN nanomembrane (see Figs 11.9b and 11.9c). The monochromatic CL images of this typical region show that the dislocation-related whiskers emit mainly yellow luminescence, while the bottom areas of the etched regions exhibit UV radiation correlating with the near-bandgap emission of GaN. The suspended

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11.9 Comparison of the SEM image (a) with monochromatic μ-CL images taken at wavelengths 365 nm (b) and 550 nm (c) as well as with two-color (365 nm and 550 nm) integral intensely image (d) taken from a selected region of a sample subjected to 0.4 keV Ar-ion pretreatment and PEC etching. The width of each image is 1.8 μm. (Reprinted from Tiginyanu I, Popa V and Stevens-Kalceff MA (2011), Membrane-assisted revelation of the spatial nanoarchitecture of dislocation networks, Materials Letters, 65 (2), 360–362. Copyright (2011) with permission from Elsevier)

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nanomembrane is characterized by both yellow and UV emissions with the dominant contribution from the yellow emission. There is likely to be a significant contribution to the UV component from the underlying etched regions due to the transmission of CL through the thin suspended nanomembrane (see monochromatic μ-CL images in Figs 11.9b and 11.9c as well as the composite μ-CL image in Fig. 11.9d). Figure 11.10 shows CL spectra taken from a dislocation cluster (curve 1), nanometer-thick suspended membrane (curve 2), and bottom area of the etched region (curve 3). The analysis of the spectra confirms that in the thin nanomembrane the intensity of the yellow emission is greater than the intensity of the near-bandgap emission. Besides, one can see that dislocations emit mainly yellow light, while UV radiation prevails in the spectrum of the bottom area of the etched regions. According to the data of previous investigations (Shalish et al., 1999; Reshchikov et al., 2001), the yellow emission from GaN is attributed to electron transitions from the conduction band or shallow donors to a deep acceptor level with a broad energy distribution, centered at 2.2 eV below the conduction band edge, the surface component being predominant in the density of yellow luminescence related states. Note that a recent study of

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11.10 CL spectra taken from a dislocation cluster (curve 1), nanometer-thick suspended membrane (curve 2) and bottom area of the etched region (curve 3). T = 300 K. (Reprinted from Tiginyanu I, Popa V and Stevens-Kalceff MA (2011), Membrane-assisted revelation of the spatial nanoarchitecture of dislocation networks, Materials Letters, 65 (2), 360–362. Copyright (2011) with permission from Elsevier)

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the spatial distribution of CL in GaN nanowires evidenced strong yellow luminescence highly localized near the surface of the nanowires (Li and Wang, 2010). We believe that in our nanomembranes the yellow component of CL is related to point defects trapping the negative charge that protects the material against PEC dissolution.

11.5

Conclusion

In conclusion, using the approach of surface charge lithography we have demonstrated the fabrication of ultra-thin GaN membranes with the thickness in the nanometer scale. The GaN nanomembranes exhibit mainly yellow cathodoluminescence and seem to possess relatively good electrical conductivity. Successfully fabricated nanometer-thin membranes open unique opportunities for exploration of the properties of 2D nanostructures based on GaN, in particular of magnetic characteristics interesting for spintronic device applications.

11.6

Acknowledgement

This work was supported by the Supreme Council for Research and Technological Development of the Academy of Sciences of Moldova under the State Programme ‘Nanotechnologies and nanomaterials’.

11.7

References

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Mei Y, Thurmer DJ, Deneke C, Kiravittaya S, Chen YF, Dadgar A, Bertram F, Bastek B, Krost A, Christen J, Reindl T, Stoffel M, Coric E and Schmidt OQ (2009), ‘Fabrication, self-assembly, and properties of ultrathin AlN/GaN porous crystalline nanomembranes: tubes, spirals, and curved sheets’, ACS NANO, 3 (7), 1663–1668. Meyer JC, Chuvilin A, Algara-Siller G, Biskupek J and Kaiser U (2009), ‘Selective sputtering and atomic resolution imaging of atomically thin boron nitride membranes’, Nano Leters, 9 (7), 2683–2689. Müller A, Neculoiu D, Konstantinidis G, Stavrinidis A, Vasilache D, Cismaru A, Danila M, Dragoman M, Deligeorgis G and Tsagaraki K (2009), ‘6.3-GHz film bulk acoustic resonator structures based on a gallium nitride/silicon thin membrane’, IEEE Electron Device Letters, 30 (8), 799-800. Novoselov KS, Geim AK, Morosov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV and Firsov AA (2004), ‘Electric field effect in atomically thin carbon films’, Science, 306 (5696), 666. Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV and Geim AK (2005), ‘Two-dimensional atomic crystals’, Proceedings of the National Academy of Sciences of the United States of America, 102 (30), 10451–10453. Park CM, Park YS, Im H and Kang TW (2006), ‘Optical properties of GaN nanorods grown by molecular-beam epitaxy; dependence on growth time’, Nanotechnology, 17 (4), 952–955. Pacilé D, Meyer JC, Girit ÇÖ and Zettl A (2008), ‘The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes’, Applied Physics Letters, 92 (13), 133107. Reshchikov MA, Morkoç H, Park SS and Lee KY (2001), ‘Yellow and green luminescence in a freestanding GaN template’, Appled Physics Letters, 78 (20), 3041–3043. Robinson JT and Perkins FK (2008), ‘Reduced graphene oxide molecular sensors’, Nano Letters, 8 (10), 3137–3140. Schedin F, Geim AK, Morozov SV, Hill EW, Blake P, Katsnelson MI and Novoselov KS (2007), ‘Detection of individual gas molecules adsorbed on graphene, Nature Materials, 6 (9), 652–655. Shalish I, Kronik L, Segal G, Rosenwaks Y, Shapira Y, Tisch U and Salzman J (1999), ‘Yellow luminescence and related deep levels in unintentionally doped GaN films’, Physical Review B, 59 (15), 9748–9751. Shukla A, Kumar R, Mazher J and Balan A (2009), ‘Graphene made easy: high quality, large-area samples’, Solid State Communications, 149 (17–18), 718–721. Singh MK, Titus E, Goncalves G, Marques PAAP, Bdikin I, Kholkinb AL and Gracio JJA (2010), ‘Atomic-scale observation of rotational misorientation in suspended few-layer graphene sheets’, Nanoscale, 2 (5), 700–708. Song L, Ci L, Lu H, Sorokin PB, Jin C, Ni J, Kvashnin AG, Kvashnin DG, Lou J, Yakobson BI and Ajayan PM (2010), ‘Large scale growth and characterization of atomic hexagonal boron nitride layers’, Nano Letters, 10 (8), 3209–3215. Splendiani A, Sun L, Zhang Y, Li T, Kim J, Chim CY, Galli G and Wang F (2010), ‘Emerging photoluminescence in monolayer MoS2’, Nano Letters, 10 (4), 1271–1275. Teweldebrhan D, Goyal V and Balandin AA (2010), ‘Exfoliation and characterization of bismuth telluride atomic quintuples and quasi-two-dimensional crystals, Nano Letters, 10 (4), 1209–1218.

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Tiginyanu IM, Ursaki VV, Zalamai VV, Langa S, Hubbard S, Pavlidis D and Föll H (2003), ‘Luminescence of GaN nanocolumns obtained by photon-assisted anodic etching’, Applied Physics Letters, 83 (8), 1551–1553. Tiginyanu I, Popa V and Stevens-Kalceff MA (2011), Membrane-assisted revelation of the spatial nanoarchitecture of dislocation networks, Materials Letters, 65 (2), 360–362. Ursaki VV, Tiginyanu IM, Volciuc O, Popa V, Skuratov VA and Morkoç H (2007), ‘Nanostructuring induced enhancement of radiation hardness in GaN epilayers’, Applied Physics Letters, 90 (16), 161908. Zhao X, Zhang Q, Hao Y, Li Y, Fang Y and Chen D (2010), ‘Alternate multilayer films of poly(vinyl alcohol) and exfoliated graphene oxide fabricated via a facial layer-by-layer assembly’, Macromolecules, 43 (22), 9411–9416. Ziegler JF, Biersack JP and Ziegler MD (2008), SRIM, the stopping and range of ions in matter, Chester, Maryland, available at: (accessed April 2011).

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12 Nanocoatings for tribological applications S. ACHANTA and D. DREES, Falex Tribology NV, Belgium and J.-P. CELIS, Katholieke Universiteit Leuven, Belgium

Abstract: An overview is given of the possible applications of nanostructured coatings for mitigating friction and wear. Different aspects of nanostructured coatings like the available deposition methods, the structure-to-property relationships, and the suitable tribological characterization techniques are reviewed. Finally, the challenges linked to the further implementation of such novel nanostructured coatings into industrial practice are critically discussed. Key words: nanomaterials, coatings and thin film technology, tribology, friction, wear, solid lubrication.

12.1

Introduction

Nanostructured materials with tuneable properties are gaining importance in many fields including tribology. Automotive, heavy machinery, cutting tools, etc. require special materials in order to function efficiently with minimum energy loss. The use of nanostructured coatings in the field of tribology, especially for harsh contact conditions, is a long-standing research topic. There is ample literature describing different coating deposition techniques, classification of nanostructured coatings, and also tribological behavior of these coatings. The superior mechanical properties offered by nanostructured coatings make them ideal tribological surfaces providing low friction and low wear loss. In this chapter, the use of nanostructured coatings as tribological surfaces for both friction and wear reduction is discussed based on some examples from state-of-the-art research. The first section of this chapter gives a general overview of common friction and wear mechanisms encountered in engineering applications. Based on this introduction, possible tribological avenues where nanostructured coatings could be beneficial are outlined. The second section is a brief review of methods used to deposit nanostructured coatings on substrates. A summary of structure–property relationships for these coatings is given along with a comparison of their mechanical and tribological properties with commonly used engineering systems. In the final section, different advanced techniques for friction and wear characterization 355 © Woodhead Publishing Limited, 2011

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of nanostructured coatings are discussed. Finally, the challenges of extrapolating laboratory experiments to field applications are discussed with the emphasis on the importance of a good tribological simulation. The chapter is concluded with some trends and possible challenges in the commercialization of nanostructured coatings for the industry.

12.2

Use of nanostructured coatings in tribology

Nanostructured coatings are already used in many settings, such as construction and exterior protection (e.g., anti-weathering coatings, paints, etc.), military and defence equipment (for wear and corrosion protection), and the energy, aerospace and automotive sectors (e.g., anti-wear coatings, corrosion, thermal barriers, hydrophobic coatings, etc.). They are also being investigated in the healthcare sector as load-bearing surfaces, and for biocompatibility, etc. The market for nanostructured coatings was valued at $980 million in 2009 and is expected to grow in the next years. The applicability of nanostructured coatings in tribology has been under investigation since year 2000.1 Nanostructured coatings in tribology are useful for (i) their superior mechanical properties, such as hardness, fracture toughness, etc., leading to an improved wear resistance, and (ii) reduction of friction by solid lubricants present in the microstructure. To understand the importance of these two aspects, a review of the basic friction and wear mechanisms follows.

12.2.1 Basic friction and wear mechanisms Tribological events like friction and wear are system properties.2 Friction and wear result from interactions at the asperity level between surfaces. Sliding materials are an integral part of a mechanical system, and the intrinsic properties of materials, such as hardness, Young’s modulus, shear strength, etc., influence friction and wear. Coatings with special properties enter into the picture, and a good understanding of basic friction and wear mechanisms allows assessment of material requirements. Friction The word ‘tribology’, derived from Greek ‘tribos’ meaning rubbing, is the study of friction, wear, and lubrication.3 Friction is a common dissipation mechanism encountered in everyday life that has both beneficial and disastrous effects. In simple terms, friction is a force that opposes motion, but this simple definition originates from a nexus of events taking place between contacting surfaces related to physical, mechanical, chemical, microstructural, and/or environmental factors.

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The earliest systematic studies of friction were carried out by Leonardo da Vinci (1452–1519) by sliding a mass M across a surface and then measuring the minimum force required for sliding.4 Most of the experiments were done on wood versus wood and wood versus iron. Based on da Vinci’s observations and later studies by Amontons5 and Coulomb,6 three classic laws of friction were postulated: 1. the friction force, Ff, is directly proportional to the applied normal force, FN, also known as ‘Amontons-Coulomb law’, or Ff = μ.FN

[12.1]

where μ is the coefficient of friction supposed to be a constant for a material couple sliding under specified conditions; 2. the friction force is independent of the apparent contact area between the sliding surfaces; and 3. the friction force is independent of the relative sliding velocity of the surfaces. The first physical explanation of friction was given by Bowden and Tabor7 for a wide variety of contact systems. The force required for rupturing adhesive bonds between surfaces was considered to be at the origin of the friction force. Considering the shear strength, τ, of the adhesive bond, then friction force, Ff, can be expressed as: Ff = τ.A

[12.2]

with A the true area of contact between the contact points. Bowden and Tabor proved the validity of this expression on plastically loaded contacts by confirming a direct relationship between applied load and electrical conductivity which is directly proportional to the intimate contact area between the surfaces. In general, the relationship between applied load and true contact area determines the evolution of the friction force. The load versus contact area dependence in the case of rough surfaces was first given by contact theories such as the Hertz8 or Greenwood–Williamson (GW) model.9 Using a statistical approach, they showed that the real contact area between surfaces varies directly with the applied load, FN. The Greenwood–Williamson theory also predicts whether the nature of the contact is elastic or plastic through a dimensionless quantity known as the plasticity index, Ψ, given by: ψ=

E* σ R H

[12.3]

with σ the effective rms roughness of the mating surfaces, E* the effective Young’s modulus, R the radius of curvature of asperity, and H the hardness

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of the surface of interest. At Ψ < 0.6, the contact is elastic irrespective of the loads used, and at Ψ > 1 the contact is plastic.9 The nature of the contact determines the friction and the wear behavior. For example, if the surfaces are in plastic contact then wear can occur easily, and friction will also change depending on the nature of wear particles. In recent years, Persson10 introduced a contact mechanics on fractal affine surfaces considering roughness at different measurement scales, confirming a direct dependence of the real contact area on the applied normal force. Using an atomic force microscope (AFM) equipped with a nanometersized probe as counterbody, tribological studies were also made at the atomic scale.11 The investigation of friction and wear at nanoscale can be done using a friction/lateral force microscope (FFM/LFM). Even at nanometer scale, the origin of friction was attributed to adhesion as governed by Eq. 12.2. Although the origin of adhesion at micro-/nanoscale is attributed to surface energy, relative humidity, etc. With LFM, the applied load and contact area dependence was found to be in agreement with different theoretical contact mechanical models. For example, Carpick and Salmeron12 observed that the friction force between a Pt-coated AFM tip and mica in ultra-high vacuum (UHV) varies proportionally with the contact area as predicted by the Johnson–Kendall–Roberts contact theory.13 They calculated the interfacial adhesive shear strength of the Pt–mica contact to be 0.3–0.9 GPa. Enachescu et al.14 reported the validity of the Derjaguin–Muller–Toporov (DMT) theory between stiff materials such as diamond-like carbon or WC–Co coatings sliding against a silicon nitride tip. The above studies highlight the importance of shear strength in the contact, and on the origin of friction. For highly loaded contacts, Eq. 12.2 was modified as: Ff = τ.A + P

[12.4]

with P the plastic deformation component or, in simple words, the force needed to displace a certain amount of material by plastic deformation or plowing. Based on Eq. 12.4, the friction force can be reduced by: • •

decreasing the shear strength of the contact; decreasing the contact area making surfaces stiffer (contact mechanical approach); • modifying the surface texture or the mechanical properties of the surface to make it hard and abrasion resistant Lowering the interfacial shear strength ‘t’ A low shearing surface layer/coating on a hard substrate can act as a low friction interface. In such a case, the load bearing is done by the hard substrate, whereas the shear forces are located in the low shearing layer.

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However, for all such lubricating layers there is an optimum thickness to obtain low friction.15 A coating thickness below optimum can cause high friction because it cannot withstand shear forces exposing the substrate, whereas at large coating thickness plowing takes place within the layer giving high friction. Some commonly used low shear metals are Ag, In, Sn, Pb, etc. Even bulk metal alloys like CoCr can act as low shear surfaces, resulting in a low coefficient of friction. Under sliding, self-mated surfaces like stellite 21 can undergo a phase transformation from bcc to hcp. Within 10 nm underneath the surface, the hcp basal planes align parallel to the sliding direction offering an easy dislocation glide.16 Other examples of solid lubricants include fluorides (CaF2, LiF2, etc.) and chalcogenides (PbS, WS2). The most common mechanism in lubricating coatings is a chemical transformation into lubricating layers followed by a transfer onto the countersurface. This is known as ‘third body’ friction because the friction-reducing layer is chemically different from the initial surfaces. The activation energy for this conversion originates from the initial friction heating, wear, and the surrounding environment. Lubricating oxides are the best example of such layers. For example, Wahl et al.17 reported through some in-situ tribometry studies that ion beam deposited Mo–S–Pb on a steel substrate, sliding against a sapphire hemisphere, forms MoS2 in the sliding contact, resulting in a low coefficient of friction of 0.05–0.17. Interestingly, the transfer of MoS2 on sapphire was more active under dry sliding conditions. Rf-magnetron sputtered BC coatings deposited on Inconel® were found to form both boric acid (H3BO3) and carbon, which also gave low coefficient of friction (0.08– 0.15) compared to 0.6–0.8 recorded on uncoated Inconel substrates sliding against sapphire. The transfer layers and trapped third bodies operate by accommodating the sliding motion within them, which Tabor termed as ‘velocity accommodation mode’.18 Diamond-like carbon coatings are another example in which the formation of graphite in the contact is responsible for a low coefficient of friction.19 In advanced applications, such as microelectromechanical systems (MEMS), special surface modification techniques are used to deposit molecular layers that have a very low surface energy, and behave in an extremely hydrophobic manner. In this way, adhesion between small structures like microgears and free standing structures can be eliminated. The nanotribology of MEMS has been under investigation since the 1990s.20 Some coatings with special nanostructured topography like poly(methylmethacrylate) (PMMA) with nanopillars are examples of such prospective materials.21 Lowering intimate contact area ‘A’ The intimate contact area between surfaces can be made small by using stiff coatings, like intermetallics, super-hard hybrid coatings, etc. In the case of

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quasi-crystalline materials, the low friction was attributed to the high stiffness.22 A study of friction recorded on Al62Cu25.5Fe12.5 quasi-crystals sliding against various materials in ambient conditions has revealed an extremely low coefficient of friction of 0.05 against diamond, 0.15 against corundum, and 0.20 against cemented carbide. Achanta et al.23 reported that Al3Mg2based intermetallics behave as low friction coatings due to their high stiffness compared to the bulk Al3Mg2 sliding under ambient conditions at 23 °C and 50% RH. Stiff coatings also exhibit low friction in LFM studies. The hypothesis is that the contact acts as a rigid spring, thereby reducing the velocity mismatch between the sliding surfaces and thus eliminating stickslip.24 For instance, a diamond tip sliding on a diamond surface in ambient conditions shows a low coefficient of friction of 0.01.22 Lowering the deformation component ‘P’ Improved mechanical properties such as hardness, fracture toughness, etc. reduce the risk of plastic deformation in the contact. The difference in hardness and other mechanical properties of the mating surfaces must be small. The contribution of the plowing component to the frictional energy loss is widely discussed in literature, and friction force can be reduced by minimizing deformation.25 Wear Wear is the loss of material due to a relative motion of two contacting surfaces, and can be expressed using a term called the ‘wear rate coefficient’. The first definition of wear rate was given by Archard for sliding contacts and is defined as the volume of material lost per unit normal load and per unit sliding distance. Thus, the wear coefficient, K, has units of mm3/N.m. Typical wear rates for metals are between 10−2 and 10−5 mm3/N.m.26 Because wear mechanisms evolve with contact size, the wear coefficient, like the coefficient of friction, is a scale dependent factor. Depending on the nature of the damage, on the factors causing wear, and on the combination of materials, wear damage can be classified into different modes as summarized in Table 12.1. The deformation mode in contact areas largely depends on the nature of the contacting materials. For contacts that are elastically loaded, wear mostly occurs by a low cycle fatigue or adhesion, whereas for plastically loaded contacts, abrasion, adhesive wear, and delamination wear have to be mostly considered. If the contact temperature is high. e.g., due to flash temperature events, wear linked to the formation of tribochemical reaction layers has to be considered both in elastically or elastoplastically loaded contacts.

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Occurs when asperities on sliding surfaces interact to form cold welds (e.g. metal–metal, metal–ceramic contact). It is characterized by high friction and the presence of lumps of material in the wear track.

Occurs when one of the sliding materials is harder than its counterpart (typically >20 %). Damage appears like regular patterns of scratches. It is common in ceramic– metal and polymer–metal combinations. Types: two-body abrasion wear, three-body abrasion wear where wear particles abrade the softer surface. Common on oxide forming materials at high temperatures. Oxide layers can be released as wear particles once a critical thickness is reached.

Adhesive wear

Abrasive wear

Corrosion wear, also known as tribocorrosion, is a common wear mechanism observed on components. The sliding action exposes blank material to the environment causing a reaction with the corrosive media. It might accelerate material degradation because materials undergo a combined mechanical and chemical attack.

Occurs in liquids due to the collapse of gas bubbles at the surface of materials. The bubble collapse results in liquid microjets directed towards the solid surface.

Corrosion wear

Cavitation wear

Fatigue wear

Occurs when hard particles or fluid droplets impinge on a surface. The damage occurs as a result of momentum transfer. The amount of wear is proportional to particle velocity, impact angle, and density of the material eroded. Erosion with particles is termed as particle erosion, whereas erosion caused by impingement of liquid droplets is called liquid erosion. Occurs when materials are subjected to cyclic stresses. This damage is common in fretting contacts which vibrate at high frequency over small displacement amplitudes. Crack initiation and propagation dominate the wear process.

Erosion wear

Oxidation wear

Description

Wear mode

Table 12.1 Different types of wear modes and examples of case studies

• Electrical contacts • With poor press-fit, bearing ring and shaft assembly • Vibrating bearings • Cam and gears • Load bearing implants • Dental bracket-wire combination • Industrial turbines • Acetabular cup femoral head in hip implants • Undersea drills and reactive materials fretting in ambient air suffering from corrosive wear • Ship propellers

• Cemented carbides (WC–Co) used on high speed cutting tools • Steam, hydroelectric turbines • Heavy-duty compressors

• Roller bearings • Unlubricated metal contacts • Starved lubrication in metallic contacts • Undersea drills • Internal combustion engine, piston skirt, cylinder liner • Gears • Roller-cone bits for rock drilling

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For the hard TiN/steel tribocouple, the load/roughness dependence has a major effect on the type of wear.27 When the plasticity index, Ψ (see Eq. 12.2) is above 1, cracking occurs in the TiN coating as well as severe plastic deformation on the steel counterbody. In contrast, when Ψ < 0.6, the TiN coatings undergo a degradation by fatigue at asperities that are loaded and unloaded during each cycle, giving rise to fine wear debris. This roughness– deformation concept is commonly applied in load-bearing implants where a high surface finish is required on the CoCr femoral head (Ra < 25 nm) to prevent plastic deformation during movements against ultra-high molecular weight polyethylene (UHMWPE) acetabular cup. Materials subjected to abrasion, erosion, and/or cavitation can become damaged in either a brittle or a ductile manner. For a ductile material, the erosion wear rate is given by:26 Q=

K 2 σ 1 / 2U 3 εC 2 H 3 / 2

[12.5]

with σ density of spherical erosive particles, εC the critical plastic strain, U the particle impact velocity, and H the hardness of the material. The erosion wear rate for ductile materials reduces with increasing initial hardness. The erosion rate for brittle ceramics is given by: Q = r 0.7U 2.4

H 0.1σ 0.2 KC 1.3

[12.6]

The erosion rate depends mainly on fracture toughness rather than on particle density or hardness. Therefore, a higher hardness alone is not sufficient to achieve a good wear resistance. The brittleness of materials can be estimated using a brittleness index which is the ratio between hardness, H, and fracture toughness, K1c, of a material (H/K1c). For example, Si has a brittleness index of 15 000 m−1/2, TiN 2000 m−1/2, and DLC 12 000 m−1/2. Silicon is thus the most brittle and prone to cracking at lower loads than TiN.28 A good balance between hardness and toughness provides a superior wear resistance, as in the case of composite materials. In recent years, a combination of hardness and elastic modulus has been recognized as an essential mechanical property in hard coatings for achieving a good wear resistance, taking into account fracture mechanisms and substrate effects. A high ratio of hardness to elastic modulus, known as the ‘elastic strain to failure index (H/E index)’, is desirable to achieve a good wear resistance of ceramic, metallic, and polymeric materials.29 In coated systems, interfacial stresses can be minimized by selecting coating and substrate materials having a similar H/E ratio. This is the case for Ti–Al–B–N and Cr–N coatings on tool steels. Nanocomposite coatings allow manipulation of the H/E

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ratio by changing the hard to soft component ratio or the phase distribution, making coatings compatible with the underlying substrate. Based on the above, a materials scientist developing a nanocomposite for tribological application would look for: • • •

•

easily shearing components in the microstructure, such as dispersed particles, secondary phases, solid lubricants, etc.; components of a coating that can form lubricious films or transfer layers; materials with high hardness but also good fracture toughness, thus an optimized microstructure with a homogeneous distribution of phases; coatings with a high H/E ratio to ensure compatibility, good adhesion, and low interfacial stresses on industrial substrates.

12.2.2 Possible avenues for application Tribological problems exist in almost every mechanical component, ranging from large gear boxes in wind turbines down to microscale gears used in MEMS. This opens a wide range where nanostructured coatings can be used as protective surfaces on a substrate or even as repair coatings. A nonexclusive list of industrial sectors where nanostructured coatings are used or investigated includes aerospace, oil and energy, automotive, microelectronics, and healthcare. Aerospace The ceramics and metals which are in common use have limitations on their operational temperatures and contact stresses. Operating temperatures in bearings of an aircraft can typically vary from −70 up to 150 °C. Even more aggressive conditions exist for spacecraft where the transition from atmosphere to space results in a broad range of operating conditions. In such applications, nanostructured materials with special adaptive surfaces can be of great benefit. Automotive Nanostructured coatings find widespread applications in the automotive industry, and their current market is around $133 million. The motor engine accounts for 50–65% of the total frictional losses.30 Polymer-based coatings containing nanosized solid lubricants, such as graphite, MoS2, PTFE, are used on piston skirt and slide bearings for friction and wear reduction. Damage to a piston skirt due to severe scuffing in an engine is shown in

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Fig. 12.1. Currently, some water-based solid bonded coatings, such as ceramic coatings containing solid lubricant phases, are used in high performance engines. This is an attractive way to make engines fuel efficient. Replacement of chromium coatings on piston rings is important due to the environmental and health-related hazards of chromium plating. In engine applications, coatings have to be compatible with oil additives. Some novel nanocomposite coatings with well-designed grain boundary composition can exhibit good compatibility with the engine oil formulation. Based on an ionic potential approach, Cu–MoN nanocomposite coatings have been developed which showed excellent compatibility with lubricant additives like zinc dialkyldithiophosphate (ZDDP), and which also possessed excellent anti-wear property.31 Such coatings are also of interest for compressors and reciprocating-type aircraft engines where lubrication is present. Some thermal spray coating systems are already in use in some automotive plants. Metal matrix composite coatings, such as cast iron containing 10% vol. Al203 and 20% vol. ZrO2 particles, are being used on some engine liners. They reduced the oil consumption by 50% and wear on the piston rings and cylinder bore by more than 30% compared to an uncoated cast iron cylinder during engine tests.32 Overall, a fuel reduction of 2–3% was obtained. This proves that coatings with better mechanical properties can enter the market. Other automotive applications include fuel injection systems, cam shafts, diesel valves, and bearings.

12.1 Damage due to scuffing on a piston skirt in a combustion engine. (Courtesy of www.mg-rover.org)

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Mechanical components Fretting is another common cause of wear in mechanical components. Most bearings operate under lubricated condition but, during starvation or boundary lubrication conditions, adhesive and abrasive wear can occur. Bad press fits during mechanical assembly also suffer from fretting, e.g., between the shaft and housing in a gear box system. Such fretting damage has been reported in wind turbines. A deleterious scenario is when debris particles, such as contaminants, sand, lubricant reaction products, etc., remain in the contact between fretted parts, and cause three-body abrasion. This is certainly an important tribological problem where novel coatings are needed. A successful example is nanostructured Al2O3 deposited by metal-organic chemical vapor deposition (MOCVD) on AISI 52100.33 Some bearing manufacturers are already using special physical vapor deposition (PVD) coatings like Ti-doped MoS2 and tungsten-doped DLC (W-DLC) for rolling and sliding contacts under poor lubrication conditions.

Turbines Components in turbines (gas, hydroelectric, or steam) that operate under extreme conditions often suffer from a combination of erosion and corrosion. There is a need for special materials suitable for high temperature, high pressure, and harsh operating conditions to combat erosive particles. A hydroelectric turbine can be subjected to erosion by silt containing hard particles carried by water. In a steam turbine, water droplets can cause erosion on impact at high velocities. Special nanostructured dispersionbased materials such as superalloy metal matrix with ceramic dispersoids,34 multilayer structures of metal nitrides and carbides are under development. For example, nanomultilayers of TiN/ZrN were found to possess an excellent wear resistance against erosion.35

Deep drilling tools WC–Co cemented carbide inserts or buttons are commonly used on drill heads, e.g., roller-cone heads for oil exploration (Fig. 12.2). The intermixing of rock with the Co binder matrix may form a new composite structure. This composite structure dictates the wear rate of the cemented carbide (Jacobson and Hogmark, 2009). Further improvements of the erosion and corrosion resistance can be made by the introduction of nanostructured WC–Co or WC–10Co–4Cr coatings. Nanostructured WC–Co coatings containing nanometer-sized WC particles in Co matrix are known to posses better tribological properties than conventional grades.36,37

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12.2 A rock drill with cemented carbide buttons. (Courtesy of www. drillbit.com)

Biomaterials Nanostructured coatings are also used as biomaterials for load-bearing applications and drug delivery or for their antibiotic properties. An example is nanostructured hydroxyapatite (HAp) coatings on implants which offer better osteointegration, a microstructure similar to that of bone, and enhanced mechanical properties. HAp coatings can be further strengthened by nanocrystalline yttria-stabilized zirconia (YSZ) dispersoids.38 In vitro studies showed that nanosized Al2O3, TiO2 ceramics give improved cell proliferation.39 Recently, plasma-sprayed HAp coatings were further reinforced with carbon nanotubes to improve fracture toughness, biocompatibility, and wear resistance.40 Apart from solid coatings, special nanocomposite gels are under investigation for their bioactivity and anti-bacterial properties. Nanocomposite gels based on chitosan with enhanced bioactivity and special load-bearing properties were recently reported as a possible solution against fretting issues.41 Repair coatings Nanostructured coatings can also be used for refurbishing worn components in an economical way rather than replacing the whole component. For example, in automotive applications, thermal spray repair of worn shifter forks, crankshafts, bearing regions, and transmission shafts has been successfully implemented.

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12.3

367

Review of nanostructured coatings for friction and wear applications

In this section, different ways to deposit nanostructured coatings, fundamental mechanisms, and structure–property relationships of different kinds of nanostructured coatings are reviewed. At the end, a comparison between nanostructured coatings and common engineering materials is made based on the available literature.

12.3.1 An insight into the structure–property relationships of nanostructured coatings The evolution of nanostructured coatings in tribology is summarized in Fig. 12.3. The research began with basic and available materials and became gradually more complex, moving on to multicomponent coatings and eventually to nanostructured smart surfaces. A good overview has been given by Donnet and Erdemir.42 The limitations of conventional solid lubricants, such as lamellar solids MoS2, WS2, graphite, etc., and soft metals, such as lead, gold, silver, etc., can be eliminated by the use of nanostructured coatings. For example, MoS2 is an excellent lubricant in vacuum but suffers from severe oxidation in ambient air. To increase its stability, MoS2 coatings can be sputtered along with metallic elements like Cr, Au, Ti.43 Another major problem with soft solid lubricants is that they wear fast. The best solution to overcome this problem is to combine them with wear-resistant hard ceramics based on B, C, N, and O. Such a combination gives rise to a special type of materials known as ‘hybrid/multicomponent coatings’. These coatings contain various types of soft/hard materials processed either by the same or different techniques. They exist in various dispersion-type forms described as nanocomposite, graded structure, and superlattice multilayers. Basic strengthening mechanisms Compared to conventional materials that have grains in the range of hundreds of microns, nanograined materials have a higher yield strength, hardness, and elastic modulus. The nanostructured materials have grain sizes in the nanometer range. As the grain size gets smaller, the ratio of atoms in grain boundaries and inside grains increases rapidly, resulting in materials with different properties. This is because on grain refinement the stress required for dislocations to propagate from one grain to another increases, giving a strengthening effect as expressed by the Hall–Petch relationship.44 The relationship between average grain size and strength for nanostruc-

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Year

1980

1990

2000

Soft coatings

• Ternary & quaternary

Basic materials

• New basic compositions • Combination with oils

Hard coatings

PVD Process

• Hybrid processes • Low lemperature • Large-scale manufacturing • Surface structuring

(PE)CVD IBAD

Single component Multicomponent, multilayer Coatings structure Nanstructured, superfattice, gradient Adaptative (smart)

12.3 Historical trends in the development of tribological coatings.42

tured materials is only valid within certain limits. As the grain size decreases further, the number of dislocations in the pile-up also decreases and, instead of pile-up, the dislocations cross grain boundaries. Thus, the dislocation strengthening mechanism evolves into grain boundary sliding below a certain grain size, supposed to be <25 nm,45 causing grain rotation and a softening effect. The relationship between hardness and grainsize is shown in Fig. 12.4. The same Hall–Petch relation was found to be valid in multilayered materials where dislocation pile-up at the layer interface has a strengthening effect in Al/AlxOy films.46 Once again, below a certain critical layer thickness, plastic flow occurs by single dislocations movement in the layers.

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Nanoscale

Hardness

Amorphous

Micro/macro scale

Grain sliding

Viscoelastic

Dislocation movement (α d–1/2) Grain size

12.4 Schematic representation of hardness as a function of grain size.44

The grain boundary area or interfacial area can also be increased by introducing secondary phases. For example, amorphous phases (a-) are added to the nanocrystalline (nc-) transition metal interstitial compounds to improve the mechanical properties, e.g., nc-TiN/a-Si3N4 coating. In the case of microstructures with dispersions or secondary phases such as precipitates, oxides, etc., two important strengthening mechanisms operate, namely, dislocation pinning by the Orowan mechanism and the Zenner pinning effect. In the Orowan mechanism, secondary particles interact with dislocations, and dispersoids <100 nm are required for it to occur.47 The other strengthening mechanism of dispersoids is by the limiting of grain boundary migration or grain growth, known as Zenner pinning. The Zenner pinning mechanism was found to be valid in dispersion-reinforced copper with grain size below 40 nm.48 Because of their microstructural stability, such materials possess good hardness combined with fracture toughness. For example, cermet ZrO2/Cu coatings with nanosized ZrO2 particles embedded in a fine-grained Cu matrix possess a better wear resistance than the conventional ones.49 Based on these fundamental strengthening mechanisms, the commonly researched nanostructured coatings are: • • • •

graded coatings; super lattice multilayer; nanocomposites; smart adaptive coatings.

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Graded coatings These coatings have one of the components deposited with a gradient from the substrate up to the surface of the coating. Such a gradient-based approach is useful to achieve a good combination of adhesion and wear resistance. Hard TiAlN with a soft MoS2 phase distributed as gradient was recently prepared using physical vapor deposition (PVD).50 A similar gradient approach was used to dope DLC with a Ti-rich gradient layer to improve the adhesion of the DLC films.51 Superlattice multilayer Multilayered coatings are the subject of research to improve mechanical, tribological, and chemical properties. The introduction of a number of interfaces parallel to the substrate can deflect cracks or provide barriers to dislocation motion, increasing the toughness and hardness of the coating. The alternating of high-modulus material and low-modulus material is known to confer high yield strength. A superlattice corresponds to the multilayer concept extrapolated to a thickness of individual layers in the range of 5–50 nm. Ti/TiN and TiN/CrN nanomultilayers were reported to exhibit an excellent thermal stability and tribological properties at 773 K. The nanomultilayers showed better properties as compared to the single-layer TiN coating. Decreasing the layer spacing from 100 to 20 nm always led to higher hardness and better scratch adhesion and wear resistance.52 The same crack deflection and stress relaxation mechanisms were observed in TiC/TiB2 multilayers.53 Similarly, a high hardness was reported in (Ti,Al)N multilayers which was attributed to their high covalent bonds and strengthening effect related to the large number of interfaces (see Fig. 12.5a).54 An example of a commercially available PVD TiN/a-SiNx multilayered coating with a 40 GPa hardness is shown in Fig. 12.5b. Nanocomposites Nanocomposites usually refers to dispersion-type coatings wherein one of the components is nanosized but remains dispersed in a matrix. A common example is coatings with a solid lubricant dispersion in a hard wear-resistant matrix or a combination of a nanocrystalline material in an amorphous matrix. In this sense, the precipitation-hardened materials can also be termed nanocomposites because the secondary phases with a size of 5–30 nm initiate a strengthening mechanism by arresting dislocations. A TEM image of a nc-TiN/a:C-H nanocomposite with 35 GPa hardness is shown in Fig. 12.6.55

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(b)

371

1 μm

25 nm

12.5 (a) Nanomultiayers of TiN/(Ti,Al)N on steel substrate with hardness of 34 GPa.54 (b) Cross-section TEM image of PVD TiN/a-SiNx nano-multilayer ultra-hard coating 40 GPa. (Courtesy of Teer coatings)

Nanostructured cermet coatings such as WC–Co are also a common example of nanocomposites which contain a high volume of grain boundaries. The large crystalline/amorphous transition across grain–matrix interface prevents crack initiation, and arrests crack growth. In recent years, coatings like ceramic ZrO2 and cermet WC–Co coatings with a nanostructure were successfully obtained starting from nanosized powders using thermal spraying techniques.56 An example of nanostructured WC–Co coatings produced by the high velocity oxy-fuel (HVOF) spraying technique is given in

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Nanocoatings and ultra-thin films A

BF A

100 Å

12.6 TEM image of nanocomposite nc-TiN/a-C:H with 35 GPa hardness.55

nc-WC-12Co

100 nm

Metal Binder

nc-WC-12Co

WC

50 nm

12.7 TEM of nanostructured WC–12Co coating produced by HVOF spraying. Dark field image of metal Co binder (left) and WC particle (right).

Fig. 12.7. In some nanocomposites, dispersed particles also impart fracture toughness by transformation toughening. The tetragonal to monoclinic phase transformation in ceria-stabilized zirconia (nanocomposite 10Ce– TZP/Al2O3) is the fracture toughness mechanism.57 The nanocomposites can be prepared to have hardness values similar to those of multilayered superlattice coatings by keeping the secondary phase in the same size range as the layer thickness (3–10 nm). TiN/Si3N4 nanocomposites were prepared by plasma-assisted chemical vapor deposition (PACVD) with 4–7 nm TiN crystals in a-Si3N4 matrix and showed a hardness of 50 GPa.58

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Smart adaptive coatings The environment has a dramatic effect on the tribological performance of any coating. To address environmental variations, a new type of coatings made of yttria, Au, MoS2, and DLC, known as ‘chameleon coating’, has been proposed.59 This type of coating was introduced for space applications where materials have to operate under space/terrestrial environmental cycles (see Fig. 12.8). Nanocomposite coatings made of YSZ in a gold matrix containing nanosized reservoirs of MoS2 and DLC, were developed using a combination of laser ablation and magnetron sputtering. Friction forces and surface reactions with the environment are used to generate a lubricious transfer film at the tribological contact that self-adjusts with each environmental change. It was reported that the surface layer switched its chemistry and structure between graphitic carbon on sliding in humid air, hexagonal MoS2 on sliding in dry N2 and vacuum, and metallic Au on sliding in air at 500 °C. The same authors also included alumina (Al2O3) in Au matrix containing DLC and MoS2 nanoparticle solid lubricants.60

12.3.2 Deposition methods for tribological applications Currently, the surface modification technologies shown in Table 12.2 are used to obtain nanostructured coatings. Most of these multicomponent and nanocomposite coatings are widely produced using chemical vapor deposition (CVD) and PVD techniques. Both methods allow deposition of relatively thin (up to tens of μm) coatings. CVD is a generic name for a group of processes that involve the deposition of a solid material on a substrate by activating precursors present in a gaseous phase, and making them react chemically. Gas phase processes,

Contact load force

Direction of sliding

Lubricious transfer film (tribo-skin)

200 nm Substrate

Hard nanocomposite Gradient interface coating with solid for load support lubricant reservoirs and stress relief

12.8 Au/MoS2/DLC/YSZ chameleon coating that reacts under varying environmental conditions.59

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Nanocoatings and ultra-thin films Table 12.2 Classification of coating deposition techniques of interest for the synthesis of nanostructured coatings75 Gas-phase processes (vapor deposition process)

Liquid-phase processes (electrochemical deposition)

Solid-phase processes (thermal sprayed coatings)

Physical vapor deposition (PVD) Chemical vapor deposition (CVD)

Electrolytic deposition Electrophoretic deposition

Air plasma spraying (APS) High velocity oxy-fuel (HVOF) spraying Cold spraying

surface chemistry, nucleation and growth phenomena play a significant role during deposition. The coating is deposited uniformly all over the substrate, including blind spots. This also allows parts to be stacked on trays. CVD coatings are usually a few microns thick and generally deposited at fairly slow rate, usually in the order of a few hundred micrometers per hour. DLC coatings are commonly deposited using CVD. In PVD, the precursors are solid with the material to be deposited being vaporized and deposited onto a substrate. The advantage of PVD surface coating technologies in large-scale and high volume operations is reduction of hazardous waste when compared to electroplating and CVD processes. Most PVD coatings are characterized by a columnar growth, and have high impact strength and an excellent scratch resistance, making them attractive as durable topcoats. Coatings such as TiAlN, TiCN, and CrAlTiN multilayers can be deposited by PVD on a variety of substrates.61 The latest PVD techniques use RF substrate holders which enhance the mobility of adsorbed atoms making it possible to get dense, homogeneous coatings at low deposition temperature. The PVD and CVD processes may induce a significant temperature increase during deposition, ranging between 100 and 1000 °C. Thus, substrates, such as cemented carbides, ceramics, and some heat-resistant steels, can be coated by high temperature CVD and PVD processes, whereas the applicability of high temperature CVD and PVD for substrates such as low alloy steel, copper-based, light-metal-based alloys, polymers still remains limited. Techniques like magnetron sputtering and ion beam assisted deposition (IBAD) are commonly used because of their lower deposition temperature. For the same reason, plasma-assisted or enhanced CVD (PACVD or PECVD) is a popular method to deposit coatings. In this process, the plasma is used to enhance chemical reactions, thereby broadening the range of substrates. For example, the ASM Handbook on Surface Engineering states that the deposition of TiN with the thermal CVD process needs 900–1000 °C while with plasma only 500 °C is required.62 Both methods are

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known to produce pure (low contaminant), dense, and homogeneous coatings, but a good understanding of the process is essential to achieve reproducible coatings. Small variations in the composition of the elements can have a significant effect on the coating microstructure and mechanical properties. An example of the growth pattern in PVD-deposited nc-TiN/ a-Si3N4 is shown in Fig. 12.9a. The best distribution and homogeneity of nc-TiN phase is obtained between 12 and 18% Si. Irrespective of the deposition method, whether PACVD or PVD magnetron sputtering, a similar 15–20% of amorphous phase is needed to achieve maximum 40–60 GPa hardness as shown in Fig. 12.9b.63 Some disadvantages of PVD and CVD techniques are: •

•

hard coatings deposited by CVD and PVD coatings may suffer from poor adhesion with the substrate material – a careful selection of the temperature range must be done to avoid interfacial stresses due to uneven thermal properties of coating and substrate; limitations on the component size to be coated and slow deposition speeds.

Electrochemical deposition is attractive because it typically takes place at low temperature. Electroplating is an electrochemical process during which metal ions are reduced on an electrically conductive substrate. It consists of oxidation-reduction reactions. Oxidation occurs at the anode, while at the cathode reduction occurs. Over the years, particularly chromium and nickel plating were applied. Recently, growing environmental concern due to carcinogenic Cr6+ ions in the case of chromium plating and the risk of nickel allergy have put a major constraint on these processes. Moreover, electroplating also suffers from toxic waste or effluents produced during operation. Nanostructured Ni was one of the early materials deposited and systematically studied. Nano-Ni coating was five times harder and 170 times more wear-resistant than coarse-grained Ni.64 A TEM image of cobalt-hardened gold electrodeposited nanastructured coating used in electrical contacts is shown in Fig. 12.10a. This coating possesses excellent wear resistance compared to coarse-grained structures. Currently, various types of coatings, such as multilayers of Cu/Ni, Ni/Ni–P, Co/Cu, Co/W, etc., metal–polymer, polymer– ceramic, metal–ceramic composites, such as Ni–PTFE, Ni–Al203, Ni–SiC, Cu/ Cu2O65 etc., can be deposited by the co-deposition of polymer/ceramic particles. An example of electrodeposited Al–SiO2 nanocomposite is shown in Fig. 12.10b.66 Avoiding agglomeration of nanoparticles is a challenge in the electrodeposition of nanocomposite coatings. Recently, Gyawali et al.67 used ultrasonic aid to prevent agglomeration and produced Ni–SiC nanocomposite coatings with superior mechanical properties. Another powerful technique for the deposition of nanostructured coatings is thermal spraying. Thermal spraying is, however, limited to the

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<111> d > 50 nm

random d ≈ 8–20 nm

A

B

TiN orientation grain size

random d ≈ 5 nm C

Growth direction 5 nm

0

5

15

10

20

Silicon content (at %) (a) 60

nc-TiN/a-Si3N4 Veprek SCT '98

50

Diserens SCT '99 Vaz SCT '00

40 Hardness (GPa)

376

30

20

10

0 0

20 40 Percentage amorphous phase

100

(b)

12.9 (a) Growth pattern in PVD unbalanced magnetron sputtered nc-TiN/a-Si3N4 nanocomposite as a function of Si content. (b) Comparison of hardness values of nc-TiN/a-C:H nanocomposites as a function of the amount of amorphous phase.63

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100 nm

(a)

Acc.V

Spot Det WD Exp

(b)15.0 kV 3.0 SE 9.1 1281

2 μm

12.10 (a) TEM microstructure of cobalt hardened gold electrodeposits. (Courtesy of KULeuven-Dept. MTM) (b) Fractured cross-section of a Al–SiO2 nanocomposite coating containing 200 nm SiO2 particles obtained from a AlCl3–DMSO2 electrolyte.66

deposition of nanocomposite coatings of the metal–ceramic, metal–metal, and ceramic–ceramic type. Plasma spraying refers to a group of spraying techniques that can be classified as atmospheric plasma spraying (APS), HVOF, vacuum plasma spraying (VPS), and high pressure plasma spraying. Out of these techniques, the HVOF and APS techniques are commonly used for the deposition of nanostrucured coatings.68 A comparison of different thermal spraying methods is given in Table 12.3. The most interesting aspect from an industrial point of view is the deposition rate which is much higher than all the available PVD, CVD, and electrochemical processes, making it ideal for coating large components and volumes.

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Table 12.3 Comparison of different thermal spraying techniques75 Thermal spraying process

Flame temperature (K)

Particle velocity (m/s)

Adhesion (MPa)

Porosity (%)

Deposition rate (Kg/hr)

Coating thickness (mm)

Flame Arc APS HVOF

∼3350 ∼6100 ∼14 000 ∼3440

40 100 200–300 600–800

<8 10–30 20–70 >70

10–15 5–10 1–8 1–2

1–10 6–60 1–5 1–5

0.2–10 0.2–10 0.2–2 0.2–2

10.0

10.0 nc WC-12Co

u WC-12Co 7.5

7.5 WC particle

WC particle 5.0

5.0

2.5

2.5

2.5 μm 0 (a)

2.5

2.5 μm 5.0

7.5

0 10.0

0 (b)

2.5

5.0

7.5

0 10.0

12.11 Contact AFM images of WC–12Co (a) conventional and (b) nanostructured produced by HVOF spraying process.75

Thermal spraying with APS or HVOF is a two-step processes involving powder preparation and spraying. The cryomilling process provides a grain refinement, and dispersoids can also be incorporated during the milling process. A comparison of conventional WC–Co coating with a nanostructured WC–Co produced by the HVOF process is shown in Fig. 12.11. Other coating systems sprayed with this technique include nanocomposites such as Al/Al2O3, FeCu/WC–Co, WC–10Co–4Cr, FeCu/Al2O3/Al, Cr3C2– NiCr, etc. The major challenges of thermal spraying are the retention of the nanostructure, the prevention of decomposition of the coating constituents, and minimizing porosity. In recent years, the cold spray or supersonic spraying technique was introduced to minimize some drawbacks of thermal spraying. Lima et al.69 and Kim et al.70 reported an improvement in adhesion and a reduction of the decomposition of WC grains in nano-WC–Co coatings deposited by the cold spray technique.

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Summary Each deposition technique has advantages and limitations. For example, CVD and PVD methods give a homogeneous and flexible microstructure, and super-hard materials, but are limited by the size of the component, deposition rate, maintenance costs, etc. On the other hand, methods like thermal spraying have the advantage of fast deposition speeds, large components, and ease of exploitation, but do not produce coatings with the same wide variety and quality as CVD, electrodeposition, and PVD. So the selection of the coating deposition technique largely depends on the application. In conclusion, all the deposition methods have the challenging tasks of controlling grain growth and the diffusion process while retaining nanostructure after deposition, and achieving this over large areas.

12.3.3 Comparison with engineering materials In this section, a comparison between the tribological properties of nanocomposite coatings and those of commonly used engineering materials is made based on a literature survey. The nanostructuring of Ni by electrochemical deposition gave a striking improvement. An optimum combination of properties was obtained at a grain size of 13 nm as shown in Fig. 12.12. The abrasion resistance of Ni also improves by more than 50%.71 Ni/ Al2O3 nanocomposites with 50 and 300 nm Al2O3 in a Ni matrix exhibit an enhancement in hardness in comparison to pure Ni.72 TiN is commonly used as a coating on cutting tools. TiN and TaN when deposited as nanomultilayers by PVD show striking improvements in erosion and abrasion wear resistance compared to TiN as shown in Fig. 12.13a. The coating with a periodicity of 11 nm gave the best performance.

700

90 μm 214 nm 62 nm

1.5 18 nm 13 nm

1.4 Wear rate (nm/cycle)

Hardness (HV)

600 500 400 300 200 100

1.2 1.1 1.0 0.9 0.8 0.7

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 d–1/2 (nm–1/2)

90 μm

1.3

214 nm 62 nm 18 nm 13 nm

0.6 0 1 2 3 4 5 6 7 8 9 10 11 12 Wear cycles (×1000)

12.12 Hardness of electrolytic Ni coating versus grain size (left), and abrasion resistance of nano- and coarse-grained Ni (right).71

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Abrasive (μm/mmN) 300

Erosive (10–2/kg SiC) Abrasive wear rate Erosive wear rate

250 200

nc-(TiAl)N/a-Si3N4

(AlTi)N

150 100

TiN

50 0

(a)

TiN

TaN

TiN/ TiN/ TiN/ TaN TaN TaN 11 nm 48 nm 220 nm

0

500 1000 1500 2000 2500 3000 Number of drilled holes

(b)

12.13 (a) Comparison of abrasion and erosion wear resistance of TiN/ TaN multilayer coatings with TiN. (b) Comparison of lifetime of cemented carbide drills coated with TiN, (Ti, Al)N, and nc-(TiAl)N/ a-Si3N4 nanocomposite.73

Similarly, cemented carbide tools when coated with PVD nc-(TiAl)N/aSi3N4 showed a 25-fold improvement in lifetime (see Fig. 12.13b) of the tool compared to conventional TiN and (TiAl)N coatings.73 Recently, Basak et al.74 compared the sliding wear resistance of various engineering materials like cast iron, lasercarb® coating, Ni–SiC, stainless steel, and conventional WC–12Co with thermally-sprayed nanocomposite coatings based on WC. They reported a clear improvement in the sliding wear and abrasion wear resistance of nanostructured WC–12Co, WC–10Co– 2Al, and WC–10Co–10Cr coatings deposited on steel substrates (Fig. 12.14). A bench test on the performance of thermally-sprayed WC–10Co–2Al nanocomposites is shown in Fig. 12.15.75 Crankshafts of a marine engine coated with either Cr plating (industrial reference) or nano-WC–Co–2Al deposited by HVOF were subjected to bench tests and the dimensional changes due to wear were recorded. Even after 750 hours of operation, the WC–Co–2Al-coated crankshaft remains intact, whereas the Cr-plated one shows a 20 μm dimensional loss. Erdemir et al.76 also developed Cu–MoN nanocomposites which were compatible with lubricant additives such as S and P. In a fired engine bench test, the Cu–MoN coated piston pins out-performed commonly used manganese phosphate coatings. Apart from the above examples, hardness and tribological data for some common nanostructured coatings are given in Table. 12.4.

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Reference coatings/ materials

381

Nanostructured coatings > 10 μm Al2O3

Wear depth (μm)

4 5N–500 μm–5 Hz–10 000 cycles 23 °C, 50 % RH

3

2 HVOF

APS

1

(0

Al

C

W

C

–C

o–

C

r– C

W

o– –C W

C

r % –C r–A C ) l( o– C 25 % r– C Al ) (5 0 % W C C –C ) o– C r

)

o–

C

no C

–C

ee –C

o(

na

71

st C W

St

ai

ni

es

s

lR

ti In

co

ne

as C

l

8

n ro

iC iS N

(L 8 71

W

In

co

ne

W

lR

C

–C

o(

co

C

)

n)

0

12.14 Comparison of wear resistance of thermally sprayed nanostructured WC–Co with industrial reference materials sliding against alumina counterbody in ambient conditions.74

Comparative test results (average values) 25 420 Dimension (mm)

25 410 25 400

Cr-coated

25 390

Nanosprayed

25 380 25 370 25 360 25 350 0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Operation time (h)

12.15 Comparison of nanostructured HVOF WC–12Co–2Al-coated and chromium-plated crankshaft in terms of dimensional change in bench test for marine diesel engines.75

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Table 12.4 Mechanical and tribological properties of some commonly reported nanostructured coatings Process and coating material

Coefficient of friction

Wear coefficient (mm3/N.m)

Countermaterial

Reference

35–45

0.45–0.60

1.1 × 10−4 against

steel

Ma et al.77

Hardness (GPa)

PVD nc-TiN/aSi3N4 PVD nc-TiN/a-BN CVD TiC/a-C:H ED Co/W

27–36

0.50–0.60

10−5

15 6.2–6.5

0.08–0.1 –

10−7–10−8 –

–

ED (TiAl)N/Mo

40–50

–

–

–

HVOF WC-Co

20–22

0.24

10−6

Alumina

12.4

Patscheider et al.63 Svensson et al.78 Tavares et al.79 Basak et al.74

Advanced techniques for characterizing tribological properties of nanostructured coatings

The tribological characterization of novel coatings requires special techniques that allow scientists to realistically simulate tribological contacts. A correct selection of tribological simulation should be based on a thorough material and mechanical analysis, complemented with a good insight into surface reactivity of materials. At a fundamental level, an understanding of the functional properties of the microstructural constituents requires advanced methods with sensitive force detection complemented by surface analytical tools. In this section, state-of-the-art tribological techniques for friction and wear characterization and major challenges in scaling up nanostructured coatings for industrial applications are discussed.

12.4.1 Techniques for friction and wear characterization Transfer mechanism and lubricious layer formation are major ways in which solid lubricants function. The key to assessing coating performance is an understanding of the formation and dynamics of the lubricating layers. There are two directions of tribological testing, namely: •

standardized test procedures like ASTM2625, AMS2488, etc. to estimate the endurance limit and lifetime of the coatings – these test procedures are used to screen the suitability of the developed materials against industrial benchmarks;

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scientific research aimed at understanding friction and wear mechanisms using advanced surface analytical techniques. In this section we limit ourselves to the scientific techniques.

With the range of tribometers that have become available since year 2000, it is now possible to quantify friction and wear from the nanoscale up to the macroscale. Such measurements over a broad range are necessary to understand the origins and scale dependence of friction and wear phenomena. Bidirectional tribological experiments at the nanoscale are done using an atomic force microscope, and the technique is called lateral or friction force microscopy (LFM/FFM). A sharp tip, typically between 1 nm and 100 nm, that simulates a single asperity is brought in direct contact with a surface and raster scanned across it over a fixed scan size (Fig. 12.16a). The cantilever deflection or torsion is recorded using laser deflection, capacitance, magnetic force, etc. For any reciprocating sliding test, a plot of friction force against sliding distance during a cycle is known as friction loop. The area of this friction loop gives a quantitative measure of the energy lost per cycle. Based on the fluctuations on the friction loop, information on topography or surface roughness and chemical/phase variations can be detected. For example, LFM is extremely sensitive to local topography and phase effects. The phase inhomogeneity causes a mirrored feature in the friction loop while the topographic effects appear as a parallel feature as shown in Fig. 12.17. A good example of phase contrasts in friction images was reported by Overney et al.80 on a silicon surface partially covered by a Langmuir–Blodgett film. In recent years, microtribometers were introduced with operating parameters that fill up the measurement gap between AFM and conventional tribometers. The measurement principle of such microtribometers is similar to the one on which LFM equipment is based but at a different normal force scale (Fig. 12.16b). Unlike in LFM, the contact sizes are larger and may range from a few μm2 up to hundreds of μm2. More information on such microtribometers can be found in Achanta et al.81 Using a microtribometer, Achanta et al.82 reported similar topography and phase effects on friction in dual-phase steel consisting of coarse austenitic grains (typical 40–60 μm) dispersed in a ferritic matrix (Fig. 12.18) similar to LFM (Fig. 12.17). Figure 12.18 shows a wear track crossing two austenitic grains in the sliding path (at locations 1 and 2) with the rest of the sliding on the ferritic matrix. Friction loops recorded at increasing sliding cycles up to 2000 cycles for the corresponding test are shown in Fig. 12.19. The friction loop corresponding to the 5th cycle exhibits some fluctuations (Fig. 12.19a). The one recorded at the 100th cycle shows at two locations in the trace and retrace direction variations in the tangential force that have mirror-like features (see locations 1 and 2 in Fig. 12.19b). On

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Nanocoatings and ultra-thin films

5 μm (a)

FOS Bending elements

S

(b)

Sample, tool Sample

Cantilever

+Z

+Y

+X

12.16 (a) Silicon nitride tip 40 nm ø used as counterbody in LFM measurements. (b) Cantilever spring element used in microtribometer.81

comparing the distance separating the locations corresponding to those mirror-like fluctuations, they appear at a separation distance of 72 μm where austenitic grains are located. The topography-phase effects noticed at nanoscale are also visible at microscale. At cycle 2000 the loops appear smooth with no features, indicating attainment of surface homogeneity due to mechanical mixing of the phases (Fig. 12.19c). In a similar way, the lubricant phases in the material also smear in the wear track and the use of such microtribological techniques helps in unraveling friction mechanisms and the lubrication behavior of coatings.

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Steps

Different phase

Topography effect Phase effect + Ft 0 – Ft

12.17 Schematic of a friction loops showing topography and phase effects.

nm

714 600 500 400

2

72 mm

300 1

Sliding path

200 100 0 –100 –200 –300 –400 –577

12.18 Wear track on etched dual phase steel after a reciprocating test performed at 20 mN normal force and displacement amplitude of 500 μm for 2000 cycles against a 2 mm ø corundum in ambient air at 23 °C and 50 % RH.82

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25

Tangential force (mN)

20

5th cycle

15 10 5

1

2

1

2

0 –5 –10 –15 –20 –25 0

100

300 400 200 Displacement (μm)

500

(a) 25

Tangential force (mN)

20

100th cycle

15

2

1

10

72 μm

5 0 –5 1

–10

2

–15 –20 –25 0

100

200 300 400 Displacement (μm)

500

(b) 25 2000th cycle

Tangential force (mN)

20 15 10 5 0 –5 –10 –15 –20 –25 0

100

300 400 200 Displacement (μm)

500

(c)

12.19 Friction loops recorded on etched dual phase steel at 20 mN normal force and 500 μm displacement amplitude at increasing sliding cycles. Sliding was done against 2 mm ø corundum in ambient air at 23 °C and 50 % RH.82

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In-situ tribometry In recent years, in-situ tribometry has opened new doors for researchers to observe the evolution of sliding contacts. A detailed explanation of in-situ methods was recently given by Sawyer and Wahl (2008).83 Raman and electron microscopy techniques are used to link real-time changes in the contact with friction and wear phenomena. In a different study, atomic-scale measurement of wear was carried out by combining a sliding test and TEM investigation to understand the role of defects and chemistry in the initiation of wear.84 In-situ tribometry measurements are valuable in understanding, the competition between different components of multifunctional nanostructured coatings made to function under varying environments. For example, in-situ and ex-situ Raman studies of the interfacial lubricating phases for MoS2/C/ Au/YSZ nanocomposites in dry and humid environments confirmed that the primary lubricant providing low friction at room temperature was MoS2 (Sawyer and Wahl, 2008).83 Similarly, the formation of MoS2 in self-mated Pb–Mo–S composite coatings was confirmed during sliding experiments. Combinatorial methods and multistation testing Modern commercial tribometers tend to be modular, so that the user could rebuild them from one configuration to another. This is done to reduce the cost of multiple equipments and speed up the analysis process. A modular tribometer of this type was recently introduced which has a pin-on-disk head, profilometer, chemical analysis, and microscopy option integrated on one machine (the DS4 tester, Tetra GmbH, Germany). In particular, the efficiency of analysis and screening is greatly improved with such modular experiments. On introducing new materials, thorough statistical information is required. This means that a number of coated samples must be subjected to testing in order to derive reliability data. New tribometers have from 50 to up to 100 stations where 100 different tribological tests can be done in one go (e.g., TE67, Pheonix tribology, UK). This is an extremely time-saving and economical way of deriving reliability information for a coating. These multistation tribometers are already used in biomaterials field85 and similar approaches are definitely of interest in screening novel nanosturctured coatings.

12.4.2 Scale dependence of tribological properties Friction and wear are highly scale dependent and the scale dependence of friction has been extensively studied. It is now accepted that the term

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‘coefficient of friction’ is not a constant but varies depending on the friction mechanism acting at a particular measurement scale. This means that the friction force varies in a non-linear way with the applied normal load in opposition to the classical Amontons’ law. In the case of bidirectional sliding tests performed over a broad range of normal forces, the friction-determining mechanisms in the case of homogeneous surfaces are shown in Fig. 12.20. The surface roughness greatly influences the friction mechanisms, especially at low normal forces and low contact sizes, while at high normal forces and large contact sizes the surface roughness is quickly lowered by destroying the asperities which give rise to wear particles. The wear mechanisms and wear coefficients also depend on the measurement scale. Like the coefficient of friction, the wear coefficient is not a constant for a given material couple. This results from the fact that as the contact size decreases, say from macro to nanometer scale, the deformation mechanisms involving excessive plastic deformation, crack propagation, delamination, fatigue, etc. do not apply anymore. At atomic scales, wear occurs by a transfer of atoms from one surface to the other, a process that is referred to as adhesive wear.86 The abrasive wear at atomic scale is defined as the dragging of atoms from one position to another under high shear forces. Keeping in mind the scale dependence of wear mechanisms, different characterization methods must be used to achieve a meaningful quantification of wear rate.

32 Mesotester

Macrotester 105 Wear loss

24 20

101

16 12

103

10–1

Geometric effects

10–3

8 4 0 10–10

10–5

Adhesion Ad A dh d hesio ion on effect effects eff ef e ffffe fec ecctt

Hertzian contact area (μm2)

Average surface roughness (nm)

Nanotester 28

10–7 10–8

10–6 10–4 10–2 Normal force (N)

100

102

12.20 Schematic representation of various friction mechanisms operating at different ranges of normal force in the case of homogeneous surfaces like DLC and TiN coatings.

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Consider a heterogeneous sample with two phases, namely matrix and solid lubricant (as in nanostructured composites with solid lubricants). Imagine a contact with area A sliding over the heterogeneous material giving rise to a friction force of Ff. If τ1 is the shear strength of the matrix and τ2 the shear strength of solid lubricant then, the friction force recorded can be expressed as Ff = τ1.α.A + τ2.(1 − α).A

[12.7]

where α is the area fraction of matrix. The equation holds under the assumption that the surfaces are very smooth: μ.FN = τ1.α.A + τ2.(1 − α).A

[12.8]

μ = μ1.α + μ2.(1 − α) = μ2 + α(μ1 − μ2)

[12.9]

where μ1 and μ2 are the coefficients of friction of the homogeneous matrix and solid lubricant, respectively, recorded over an area A under similar conditions. Based on Eq. 12.9, the overall friction depends on the contact size and the volume fraction of the solid lubricant phase. This illustrates that in inhomogeneous materials, friction and wear data can vary from location to location. Therefore, the selection of appropriate test parameters should minimize the scale dependence of tribological phenomena caused by microstructural effects.

12.4.3 Challenges to establish scale up The scale up of nanostructured coatings prepared at laboratory scale to the industry is a challenging task. The most important factors that must be addressed before commercializing the coatings are as follows.

Ability to coat different sizes of samples and retention of nanostructure The required thickness of a particular coating depends on the application. Sometimes a thin coating of a few μm is needed to protect the components, whereas other applications may require a larger thickness. Apart from this, the coating technology should be capable of coating complex shapes such as curved surfaces, etc. Conventional PVD, CVD coatings are typically limited to tens of μms before internal stresses affect the adhesion and mechanical integrity of the coated systems. Thermal spray and cold spray deposition methods allow a large thickness in the range of mm. The coating technology must be thoroughly optimized to achieve a homogeneous structure throughout the sprayed region.

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The retention of nanoscale over large surface areas is a challenging task. The deposition parameters used for lab-scale deposition of nanostructured coatings can fail when it comes to large industrial surfaces. Perform relevant laboratory tests, reliability analysis, to convince the end users, e.g., good lab-scale simulations of tribology representative of the field component It is not always evident that the coatings which perform exceptionally in lab-scale simulations also perform well in field tests. There are reports in which super-hard coatings were found to fail when used on components.29 This is an unwanted scenario and shows how lab-scale simulations can be misleading if the simulation conditions don’t comply with the application. Many of the industrial sectors are rather conservative and the introduction of newer technology can meet with fierce resistance. The ‘fear of failure’ is natural because both reputation and economical factors come into the picture. This means that a great effort is needed to convince end users with reliable data and lab-scale simulations. The tribosystem usually consists of two materials in contact with each other and with a relative motion, together with the environment in which they operate. This environment can be as simple as a single fluid (lubricant, water) or can be complex and dynamic (changing through the lifetime of the components). Apart from the physical environment, other environmental parameters play a role in the tribological behavior, such as temperature, vibrations, acoustic waves, contamination (foreign particles or liquids), etc. In a tribosystem, the materials are interacting with each other under mechanically described parameters such as speed and contact load but can also have chemical and electrochemical interactions. An electrochemical reaction will happen and will influence the friction and wear behavior, because both mechanisms are surface related. As a result, just the description of a tribosystem can be very complex. As a tribological system is complex, it is a challenge to design a good laboratory test that takes into account all the system properties. This is often a meticulous exercise and requires thorough assessment based on the following factors: • • • • •

correct mechanical simulation based, e.g., on the use of TAN number;87 the required results in terms of tribometrics; correct contact pressures; feasibility of accelerated testing; wear evolution rather than absolute wear.

Apart from a good laboratory simulation, a reliability analysis and a mapping of wear mechanisms must be done on the coating systems against

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industrial benchmarks. With such extensively generated data, the confidence of the users can be gained for marketing these novel coatings. Balance cost/performance ratio that would encourage integration of the technology in the production line A major hurdle in introducing nanostructured coatings is the cost factor. Because new technologies are in general expensive in terms of developmental costs, the developed coatings should give an adequate performance compared to contemporary materials. Therefore, the invented materials should have an optimum cost/performance ratio to be introduced into the production lines of companies. In some cases, the cost is not a factor as compared to the reliability or safety of the component (e.g., in space shuttles, airplanes, production lines, etc.). In recent years, thermal-sprayed coatings have been successfully introduced in the automotive industry for engine liners.88 Such examples are encouraging, and the same approach should be used for introducing new coatings into the market.

12.5

Conclusions and future trends

We live in a world where ‘saving energy’ and ‘cost cut down’ are two important notions. The tribological issue involves both, i.e., friction = energy loss and wear = increase in maintenance costs. Both friction and wear can only be mitigated, never eliminated. Nanostructured coatings serve as good alternatives to the conventional materials thanks to their superior mechanical and tribological properties. Industrial data on the performance of nanostrucutured coatings in the field are still scarce. More tribological data illustrating their superior reliability compared to current industrial benchmarks are needed to establish confidence in the technology among end users. The search for optimum materials with multifunctional properties will continue. Features such as self-healing, smart coatings capable of adjustment based on tribological needs, and compatible surfaces with an affinity towards lubricant additives are some promising research avenues.

12.6

Acknowledgements

Part of the information presented in this chapter has been obtained within the European FP6 research project ‘Nanospraying’ contract no. G5RD-CT2002-00862, the European FP7 research project ‘Supersonic’ CP-IP 228814-2 and the scientific community on Surface Modification of Materials funded by Science Foundation Flanders (WOG). Special thanks to Mr James Grebmeier (Schlumberger, France), Dr Marc van Drogen (SKF, The Netherlands), Dr Xiao Bo (SKF, The Netherlands), Mr Ravi Kiran (Smith

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Bits, USA), and Mr Philippe Lambert (Medacta, Switzerland) for their valuable inputs.

12.7

References

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20 Bhushan B (1999), Handbook of Micro/nanotribology (2nd ed), Boca Raton, FL: CRC Press. 21 Arvind S R, Yoon E S, Kim J H, Kim J, Jeong H E and Suh K Y (2007), ‘Replication of surfaces of natural leaves for enhanced micro-scale tribological property’, Mater Sci Eng C, 27, 875–879. 22 Persson B N J, Carbone G, Samoilov V N, Sivebaek I M, Tartaglino U, Volokitin A I and Yang C (2007), ‘Contact mechanics, friction and adhesion with application to quasicrystals’, in Meyer E, Gnecco E (eds), Fundamentals of Friction and Wear on the Nanoscale, Berlin: Springer, 269–306. 23 Achanta S, Feuerbacher M, Grishin A, Ye X and Celis J P (2010), ‘On the mechanical and tribological behavior of Al3Mg2 complex metallic alloys as bulk and as coating’, Intermetallics, 18, 2096–2104. 24 Gnecco E, Bennewitz R, Gyalog T, Loppascher Ch, Bammerlin M, Meyer E and Gunterodt H J (2000), ‘Velocity dependence of atomic friction’, Phys Rev Lett, 84, 1172–1175. 25 Buckley D H (1981), in Surface Effects in Adhesion, Friction, Wear and Lubrication, Amsterdam: Elsevier, 54. 26 Hutchings I M (1992), Tribology: Friction and Wear of Engineering Materials, London: Edward Arnold. 27 Vancoille E, Celis J P, Roos J R and Stals L M (1992), Modelling a ball-on-disk experiment for the system 100 Cr6 Steel vs. (Ti,X)N coating, in Dowson D, Taylor C M, Childs T H C and Godet M (eds), Proc. 18th Leeds-Lyon Symposium on Wear Particles: from the Cradle to the Grave, Amsterdam: Elsevier. 28 Achanta S and Celis J P (2010), ‘On the scale dependence of coefficient of friction in unlubrication sliding contacts’, Wear, 269, 435–442. 29 Leyland A and Matthews A (2000), ‘On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behaviour’, Wear, 246, 1–11. 30 Dahotre N B and Nayak S (2005), ‘Nanocoatings for engine applications’, Surf Coat Technol, 194, 58–67. 31 Öztürk A, Ezirmik K V, Kazmanlı K, Ürgen M, Eryılmaz O L and Erdemir A (2008), ‘Comparative tribological behaviors of TiN-, CrN- and MoN-Cu nanocomposite coatings’, Tribol Int, 41, 49–59. 32 Gerard B (2006), ‘Application of thermal spraying in the automobile industry’, Surf Coat Technol, 201, 2028–2031. 33 Bressan J D, Battiston G A, Gerbasi R and Daros D P (2006), ‘Wear on SAE 52100 with Nanocoating Al2O3 by MOCVD Process’, Adv Sci Technol, 45, 1336–1341. 34 Anand K, Bruce R W, Sampath S, Srinivasan D and Gray D M (2007), Wear Resistant Coatings, General Electric Company, US Patent No. 2007/0099027 A1. 35 Ulrich S, Ziebert C, Stüber M, Nold E, Holleck H, Göken M, Schweitzer E and Schloßmacher P (2004), ‘Correlation between constitution, properties and machining performance of TiN/ZrN multilayers’, Surf Coat Technol, 188–189, 331–337. 36 Zhu Y C, Yukimura K, Ding C X and Zhang P (2001)‚ ‘Tribological properties of nanostructured and conventional WC–Co coatings deposited by plasma spraying’, Thin Solid Films, 388, 277–282.

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37 Kumari K, Anand K, Bellacci M and Matteuchi M (2010), ‘Effect of microstructure on abrasive wear behavior of thermally sprayed WC–10Co–4Cr coatings’, Wear, 268, 1309–1319. 38 Ahn E S, Gleason N J, Nakahira A and Ying J Y (2001), ‘Nanostructure processing of hydroxyapatite-based bioceramics’, Nano Lett, 1(3), 149–153. 39 Lee S, Morillo C, Olivares J, Kim S, Sekino T, Niihara K and Hockey B J (2003), ‘Tribological and microstructural analysis of Al2O3/TiO2 nanocomposites to use in the femoral head of hip replacement’, Wear, 255, 1040– 1044. 40 Balani K, Chen Y, Harimkar S P, Dahotre N B and Agarwal A (2007), ‘Tribological behavior of plasma-sprayed carbon nanotube-reinforced hydroxyapatite coating in physiological solution’, Acta Biomater, 3, 944–951. 41 Travan A, Donati I, Marsich E, Bellomo F, Achanta S, Toppazzini M and Semeraro S et al., (2010), ‘Surface modification and polysaccharide deposition on BisGMA/TEGDMA thermoset’, Biomacromolecules, 11(3), 583–592. 42 Donnet C and Erdemir A (2004), ‘Historical developments and new trends in tribological and solid lubricant coatings’, Surf Coat Technol, 180–181, 76–84. 43 Ding X, Zeng X T, He X Y and Chen Z (2010), ‘Tribological properties of Crand Ti-doped MoS2 composite coatings under different humidity atmosphere’, Surf Coat Technol, 205, 224–231. 44 Pande C S, Masumura R A and Armstrong R W (1993), ‘Pile-up based HallPetch relation for nanoscale materials’, Nanostruct Mater, 2, 323–331. 45 Subramanian P R, Corderman R R, Amcherla S, Oruganti R, Angeliu T M, Anand K et al., (2004) Nanoparticle dispersion-reinforced metallic systems, in Senkov O N (ed.), Metallic Materials with High Structural Efficiency, Dordrecht, Boston, London: Kluwer Academic, 55–66. 46 Holmberg K, Matthews A and Ronkainen H (1998), ‘Coatings tribology–contact mechanisms and surface design’, Tribol Int, 31, 107–120. 47 Raynor D and Silcock J M (1970), ‘Strengthening mechanisms in precipitating alloys’, Met Sci, 4, 121–130. 48 Dieter G E (1961), Mechanical Metallurgy, New York: McGrawHill. 49 Lang S, Beck T, Schattke A, Uhlaq C and Dinia A (2004), ‘Characterisation of nanostructured coatings based on oxides for tribological applications’, Surf Coat Technol, 180–181, 85–89. 50 Efeoglu I (2007), ‘Deposition and characteriztion of a multilayered-composite solid lubricant coating’, Rev Adv Mater Sci, 15, 87–94. 51 Donnet C, Fontaine J, Le Mogne T, Belin M, Heau C, Terrat J P et al. (1999), ‘Diamond-like carbon-based functionally gradient coatings for space tribology’, Surf Coat Technol, 120–121, 548–554. 52 Srinivasan D, Kulkarni T and Anand K (2007), ‘Thermal stability and hightemperature wear of Ti-TiN and TiN-CrN nanomultilayer coatings under selfmated conditions’, Tribol Int, 40, 266–277. 53 Holleck H and Lahres M (1991), ‘Two-phase TiC/TiB2 hard coatings’, Mater Sci Eng A, 140, 609–615. 54 Carvalho N J M and De Hosson J Th M (2006), ‘Deformation mechanisms in TiN/(Ti, Al) N multilayers under depth-sensing indentation’, Acta Mater, 54, 1857–1862.

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55 Barshilia H A, Surya Prakash M, Sridhara Rao D V and Rajam K S (2005), ‘Superhard nanocomposite coatings of TiN/aC prepared by reactive DC magnetron sputtering’, Surf Coat Technol, 195, 147–153. 56 Jia K and Fischer T E (1997), ‘Sliding wear of conventional and nanostructured cemented Carbides’, Wear, 203–204, 310–318. 57 Nawa M, Nakamoto S, Sekino T and Niihara K (1998), ‘Tough and strong Ce-TZP/alumina nanocomposites doped with titania’, Ceram Int, 24, 497–506. 58 Veprek S and Reiprich S (1998), ‘A concept for the design of novel superhard coatings’, Thin Solid Films, 317, 449–454. 59 Voevodin A A, Fitz T A, Hu J J and Zabinski J S (2002), ‘Nanocomposite tribological coatings with “chameleon” surface adaptation’, J Vac Sci Technol A, 20, 1434–1444. 60 Baker C C, Hu J J and Voevodin A A (2006), ‘Preparation of Al2O3/DLC/Au/ MoS2 chameleon coatings for space and ambient environments’, Surf Coat Technol, 201, 4224–4229. 61 Andersen K N, Bienk E J, Schweitz K O, Reitz H, Chevallier J, Kringhøj P and Bøttiger P (2009), ‘Deposition, microstructure and mechanical and tribological properties of magnetron sputtered TiN/TiAlN multilayers’, Surf Coat Technol, 123, 219–226. 62 ASM (1994), ASM Handbook: Volume 5 Surface Engineering, Materials Park, OH: ASM International, 1485–1500. 63 Patscheider J A, Zehnder T and Diserens M (2001), ‘Structure-performance relations in nanocomposite coatings’, Surf Coat Technol, 146–147, 201–208. 64 Gurrappa I and Binder L (2008), ‘Electrodeposition of nanostructured coatings and their characterization – a review’, Sci Technol Adv Mater, 9, 043001–11. 65 Eskhult J (2007), Electrochemical Deposition of Nanostructured Metal/MetalOxide Coatings, Ph.D Thesis, Uppsala University. 66 Fransaer J, Leunis E, Hirato T and Celis J-P (2002), ‘Aluminium composite coatings containing micrometre and nanometre-sized particles electroplated from a non-aqueous electrolyte’, J Appl Electrochem, 32, 123–128. 67 Gyawali G, Cho S H, Woo D H and Lee S W (2010), ‘Electrodeposition of Ni-SiC nano composite in presence of ultrasound’, Mater Sci Forum, 658, 424–427. 68 ASM (1994), ASM Handbook: Volume 5 Surface Engineering, Materials Park, OH: ASM International, 1447–1450. 69 Lima R S, Karthikeyan J, Kay C M, Lindermann J and Berndt C C (2002), ‘Microstructural characteristics of cold-sprayed nanostructured WC-Co coatings’, Thin Solid Films, 416, 129–135. 70 Kim H J, Lee C H and Hwang S Y (2005), ‘Fabrication of WC-Co coatings by cold spray deposition’, Mater Sci Eng A, 391, 243–248. 71 Jeong D H, Palumbo G, Aust K T and Erb U (2001), ‘The effect of grain size on the wear properties of electrodeposited nanocrystalline nickel coatings’, Scripta Mater, 44, 493–499. 72 Simunovich D, Schlesinger M and Snyder D D (1994), ‘Electrochemically layered copper nickel nanocomposites with enhanced hardness’, J Electrochem Soc, 141, 10–11. 73 Jehn H A (2000), ‘Multicomponent and multiphase hard coatings for tribological applications’, Surf Coat Technol, 131, 433–440.

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74 Basak A K, Achanta S, Matteazzi P, Vardavoulias M, Celis J P and DeBonte M (2007), ‘Effect of Al and Cr addition on tribological behaviour of HVOF and APS nanostructured WCCo coatings’, Trans Inst Met Finish, 85, 1–7. 75 Basak A K (2009), Tribology and tribocorrosion of thermally sprayed nanostructured cermet coatings, Ph.D Thesis, K.U.Leuven. 76 Erdemir A, Eryilmaz O, Orgen M and Kazmanli K (2008), Development of multifunctional nanocomposite coatings for advanced automotive applications, Proc. 16th International Colloquium Tribology, 15–18 January, Esslingen, Germany. 77 Ma S, Procházka J, Karvánková, Ma Q, Niu X, Wang X, Ma D, Xu K and Veprek S (2005), ‘Comparative study of the tribological behaviour of superhard nanocomposite coatings nc-TiN/a-Si3N4 with TiN’, Surf Coat Technol, 194, 143–148. 78 Svensson M, Wahlström U and Holmbom G (1998), ‘Compositionally modulated cobalt-tungsten alloys deposited from a single ammoniacal electrolyte’, Surf Coat Technol, 105, 218–223. 79 Tavares C J, Rebouta L, Rivière J P, Pacaud J, Garem H, Pischow K and Wang Z (2001), ‘Microstructure of superhard (Ti, Al) N/Mo multilayers’, Thin Solid Films, 398–399, 397–404. 80 Overney R M, Meyer E, Frommer J and Brodbeck D, et al. (1992) ‘Friction measurements on phase-separated thin films with a modified atomic force microscope’, Nature, 359, 133–135. 81 Achanta S, Drees D, Celis J P and Anderson M (2007), ‘Investigation of friction in the meso normal force range on DLC and TiN coatings’, J ASTM Int, 4, 1–12. 82 Achanta S, Liskiewicz T, Drees D and Celis J P (2009), ‘ Friction mechanisms at the microscale’, Tribol Int, 42, 1792–1799. 83 Sawyer G W and Wahl K (2008), ‘Observing interfacial sliding processes in solid-solid contacts’, MRS Bulletin, 33, 1–4. 84 Dickinson J T (2007), ‘Single asperity nanoscale studies of tribochemistry’, in Gnecco E, Meyer E (eds), Fundamentals of Friction and Wear on the Nanoscale, Berlin: Springer, 481–520. 85 Saikko V (2003), ‘Effect of lubricant protein concentration on the wear of ultrahigh molecular weight polyethylene sliding against a CoCr counterface’, J Tribol, 125, 638–643. 86 Colasco R (2007), Surface damage mechanisms: from nano- and microcontacts to wear of materials, in Gnecco E, Meyer E (eds), Fundamentals of Friction and Wear on the Nanoscale, Berlin: Springer, 453–480. 87 Drees D and Celis J P (2000), ‘Intelligent test selection using tribological aspect number’, Proc. 12th International Tribology Colloquium, 12–14 January, Esslingen, Germany, 409–411. 88 G Barbezat (2005), ‘Advanced thermal spray technology and coating for lightweight engine blocks for the automotive industry’, Surf Coat Technol, 200, 1990–1993.

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13 Self-cleaning smart nanocoatings J. O. CARNEIRO, V. TEIXEIRA, P. CARVALHO, S. AZEVEDO and N. MANNINEN, University of Minho, Portugal

Abstract: This chapter describes the major features of current smart, self-cleaning photocatalytic materials. These materials include semiconductor materials such as titanium dioxide (TiO2). This chapter focuses on TiO2-based materials because they are most widely studied. Characteristics such as low toxicity, high chemical stability, availability and low cost make TiO2 the ideal candidate for industrial applications. Key words: titanium dioxide (TiO2), photocatalysis, superhydrophilicity, self-cleaning, smart applications.

13.1

Introduction: TiO2 photocatalysis

Since the discovery of the photocatalytic properties of some semiconductor materials, different products have been gradually introduced in the market. This development dates back to the 1990s when the number of research studies resulting in patent applications increased considerably (Paz, 2010; Carp et al., 2004). According to the report ‘Photocatalyst: Technologies and Global Markets’, the global market of photocatalytic products was $848 million in 2009 and was predicted to grow to $1.7 billion by 2014 (Gagliardi, 2010). The field of photocatalysis emerged about 80 years ago through research into the chalking and degradation of outdoor TiO2-based paints (Mills and Hunte, 1997; Fujishima et al., 2008). However up until 1960, scientific studies did not produce any product of commercial interest using TiO2 as a photoactive material (Paz, 2010). Nevertheless, these research studies provided the foundation for its eventual commercial applications. Among different photocatalytic semiconductors based on oxides and sulphides, such as titanium dioxide (TiO2), zinc oxide (ZnO), tungsten oxide (WO3) and cadmium selenide (CdSe), TiO2 has received the greatest attention, due to its higher photocatalytic activity, chemical stability, availability and low cost (Hoffmann et al., 1995). During the 1960s A. Fujishimabegan to study the photo-electrolysis of water using a TiO2 semiconductor electrode to oxidize water to oxygen. In 1969 he and his co-workers demonstrated, for the first time, the electrochemical photolysis of water using TiO2 (Hashimoto et al., 2005). This work 397 © Woodhead Publishing Limited, 2011

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was reported in 1972 in Nature (Fujishima and Honda, 1972), and started a revolutionary change in photo-electrochemistry (Paz, 2010). In response to the oil crisis of the 1970s, research on TiO2 photocatalysis for hydrogen production was undertaken to the mid-1980s. Despite the high photocatalytic efficiency of TiO2, it was found to function best irradiated by UV light, making it unattractive for H2 production. Academic and industrial research shifted to the application of TiO2 to photodegradation of pollutants (Hashimoto et al., 2005). This new use was first proposed in 1977 for water purification (Lia et al., 2006). In subsequent years, the detoxication of dissimilar compounds in water and air was achieved by using powdered TiO2. During the 1990s, researchers noticed that TiO2 was not efficient in the treatment of large quantities of water and air, since UV radiation only corresponds to a small fraction of solar light and/or artificial light sources. Fujishima and his coworkers began to study different applications which only required a small amount of UV light-promoting photon-induced reactions on a TiO2 surface. Research on TiO2 photocatalysis now focused on the material’s selfcleaning properties. The first reports on photocatalytic cleaning materials date to 1992, with a self-cleaning ceramic tile coated with TiO2 developed by Fujishima et al. Fujishima and his co-workers found huge differences in TiO2 water wettability before and after UV light exposure. They reported that UV radiation exposure promoted a considerable reduction in water contact angle, which resulted in a non-water-repellent surface. This became known as superhydrophilic behavior. They also reported that hydrophilicity was kept for 1–2 days without the presence of UV radiation. After this period of time, without a UV source, the water contact angle slightly increases, reducing hydrophilic properties. These results made TiO2 even more attractive for self-cleaning applications, once it is possible to achieve a cleaning effect without UV light for a small period of time (Fujishima et al., 2000). Currently, the self-cleaning behavior based on TiO2 surfaces is explained by two main concepts: super-hydrophilic surfaces (water contact angle of about 0°) and super-hydrophobic surfaces, showing contact angles nearly of 150°. These surface states have been mainly achieved through the tailoring of their roughness and surface energy (adhesion work) (Feng et al., 2002). At present, self-cleaning surfaces based on the photocatalytic activity of TiO2 are applied in many areas such as buildings, road paving, vehicle sideview mirrors, lamps and even in textiles (Agrios and Pichat, 2005). The collaboration of Fujishima (and co-workers) with TOTO Co. has significantly advanced the photocatalytic TiO2 market (Mills and Lee, 2002). In 1998 TOTO Co. developed HydrotectTM, which has been used for photocatalytic surfaces on tiles, glass and aluminium panels in more than 5000 buildings in Japan. The coatings keep surfaces clean for more than 20 years,

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while uncoated surfaces need to be washed every five years, representing a considerable maintenance cost (TOTO, 2011). Since year 2000, many more companies have emerged in this sector.

13.2

Photocatalysis processes

Photocatalysis is generally defined as the catalysis of a photochemical reaction at a solid surface, usually a semiconductor (Heller, 1995; Mills and Hunte, 1997; Fujishima and Zhang, 2006; Fujishima et al., 2008). Owing to their electronic structure, which is characterized by a filled valence band and an empty conduction band, semiconductors can act as sensitizers for light-induced redox processes (Banerjee et al., 2006). When a photon impinges a semiconductor with energy equal to or higher than the respective energy band-gap (Eg), an electron is promoted from the valence to the conduction band, creating a hole in the valence band. In semiconductors, a portion of these photo-excited electron/hole (e−/h+) pairs diffuse to the surface of the catalytic particle and electron/hole pairs are trapped at the surface. These (e−/h+) pairs take part in chemical reactions with the adsorbed donor or acceptor molecules. The holes can oxidize donor molecules whereas the conduction band electrons can reduce appropriate electron acceptor molecules. Figure 13.1 provides a schematic representation of these photo-excitation mechanisms. For a specific semiconductor to undergo photoinduced electron transfer to adsorbed particles, the potential redox level of the acceptor species must, because of thermodynamic requirements, be below the conduction band of the semiconductor. In the same way, the potential level of the donor species needs to be above the valence band position of the semiconductor in order to be oxidized by the holes (Stamate and Lazar, 2007). This means that the energy level at the bottom of conduction band determines the reduction

hν

Energy

Conduction band

O2–→O2–

Eg H2O/OH–→OH• Valence band

13.1 Operation of a photochemical excited TiO2 particle. (Adapted from Benedix et al., 2005).

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ability of photo-electrons and the energy level at the top of valence band determines the oxidizing ability of the photoholes (Banerjee et al., 2006). Metal oxides are ideal catalysts for solar-driven photocatalytic applications due to their low cost and high stability in aqueous solution (Alexander et al., 2008; Carneiro et al., 2007; 2008; Osterloh, 2008). The band-edge positions of several metal oxides are shown in Fig. 13.2. The positions are derived from the flat band potentials in a contact solution of an aqueous electrolyte at pH = 0. The pH of the electrolyte solution influences the band edge positions of the different semiconductors compared to the redox potentials for the adsorbate (Linsebigler et al., 1995). Heterogeneous photocatalytic oxidation by TiO2-based materials makes these materials particularly attractive in comparison to other oxidizing contaminant processes (Stamate and Lazar, 2007). TiO2 heterogeneous photocatalysis involves two different reactions that occur simultaneously. The first one is the initial oxidation promoted by the photogenerated holes and the second is the reduction by photogenerated electrons. These two supracited processes mean that the photocatalyst itself does not undergo considerable change (Fujishima et al., 2008). A heterogeneous photocatalytic system is based on the presence of semiconductor particles that are in close contact with a liquid or a reactive gaseous medium. The photo-induced molecular transformations or chemical reactions that take place at the catalyst’s surface depend on where the initial excitation occurs; they can thus be divided into two classes (Linsebigler et al., 1995). The so-called sensitized Photoreaction happens when the initial photo-excitation occurs at the photocatalyst’s surface and energy or an electron is transferred into the molecule’s ground state. However, if the photo-excitation process occurs in molecules adsorbed at the catalyst’s surface (interacting with it the ground state), the process is termed catalyzed photoexcitation.

V –2 –1 0

TiO2 Nb2O5 ZnO   Fe2O3 SnO2 H2/ H2O Lower edge position

1 O2/ H2O

of conduction band

2 3 4

Upper edge position of valence band

13.2 Band-edge energies of typical semiconductors metal oxides. (Adapted from Benedix et al., 2005).

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The general mechanism of photochemical catalysis using TiO2 under UV light irradiation (wavelength less than 385 nm) follows several stages (Hoffmann et al., 1995; Stamate and Lazar, 2007). The basic standard photochemical reactions can be written as follows (Yu et al., 2000; Schrank et al., 2002): •

photo generation electron/hole pairs υ − + TiO2 ⎯h⎯ → ecb + hvb

•

[13.1]

formation of super-oxide radicals − • − ecb , surf + O2 ( ads ) → O2

•

[13. 2]

formation of hydroxyl radicals + • + hvb , surf + H 2 O( ads ) → HO( ads ) + H

[13. 3]

TiO2 can also photodegrade organic compounds through dissimilar oxidation reactions that lead to the formation of innocuous substances such as carbon dioxide and water products. The above chemical reactions can be extended to organic materials and/or biomicroorganisms (Banerjee et al., 2006): HO(•ads) + organic compounds → → x CO2 + y H 2 O •

[13.4]

− 2

Hydroxyl radicals (HO ) and super-oxide ions (O ) are highly reactive species that will oxidize organic compounds adsorbed on the semiconductor surface. The number and lifetime of (e−/h+) pairs are particle size dependent (Shah et al., 2002). For large particles, the (e−/h+) pair’s volume recombination is the dominant process. For small sized particles, the distance covered by (e−/h+) pairs (during their trajectory from crystal interface to the surface) is short, increasing the migration rate to the surface in order to take part in the chemical reaction. Besides the effect of particle size on photocatalytic activity, the role of a metal ion dopant is also very important because it can act as an electron trap in the semiconductor interface. The trap of charge carriers can decrease the volume recombination rate of (e−/h+) pairs and thus increase the lifetime of charge carriers. The process of charge trapping can be described as follows (Shah et al., 2002): − M n + + ecb → M ( n − 1)+

[13.5]

+ M n + + hvb → M ( n + 1)+

[13.6]

+ OH − + hvb → OH•

[13.7]

where Mn+ is the metal ion dopant. The energy level of Mn+/M(n−1)+ lies below the conduction band edge. The energy level of transition metal ions thus affects trapping efficiency. Electron trapping makes it easy for holes to

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transfer onto the TiO2 surface, reacting with OH− to form active hydroxyl radicals (HO•) that will participate in the overall degradation of organic compounds (Carneiro et al., 2005).

13.3

The photocatalytic cleaning effect of TiO2-coated materials

The development of self-cleaning surfaces has followed several directions. In general, surfaces have been modified through the application of surfactants to induce hydrophilic properties so that the use of a water stream would be sufficient to remove stains and soiling caused by organic compounds. The aim was to develop permanently active surfaces. However, the lack of durability, hardness and weather resistance have been major restrictions to the large-scale use of these customized surfaces. Heterogeneous photocatalysis, on the other hand, is a promising potential technology for self-cleaning (Ramirez et al., 2010). Indeed, one of the first commercial products using a photocatalytic mechanism was coated selfcleaning glasses for tunnel lighting in Japan. Uncoated lamps tend to lose brightness due to contaminants from vehicle exhaust gases that are adsorbed onto the lamp exterior. Sodium lamp glasses coated with TiO2 semiconductor emit enough UV light (~3 mW/cm2) to allow catalytic reactions and photodecompose adsorbed contaminants (Fujishima et al., 2000). The automotive industry (glasses), chemical industry (paints), textile industry (TiO2 nanoparticles to treat effluents from processing), ceramic industry (tiles with bactericidial effect), construction industry (facade tile materials, glass and pavements) and the photovoltaic industry (application of self-cleaning glasses to optimize optical transmission) are some examples (Agrios and Pichat, 2005). Japan has been the pioneer in the development and commercialization of photocatalytic products, particularly in the collaboration between Dr A. Fujishima and co-workers in collaboration with TOTO Co. (Mills and Lee, 2002; TOTO, 2011). The wetting of a solid with water (Barthlott and Neinhuis,1997) is dependent on the relation between the interfacial tensions (water/air, water/solid and solid/air). The ratio between these tensions determines the contact angle q between a water droplet on a given surface. A contact angle of 0° means complete wetting, and a contact angle of 180° corresponds to complete non-wetting. For contact angles above 120°, the surface’s wettability state is considered to be super-hydrophobic (an example can be seen in Fig. 13.3). In general, the higher the angle the lower is the value of the adhesion work (surface energy). Hydrophobic surfaces with low wettability and contact angles of about 90° ≤ q ≤ 120° have been known for a long time (Barthlott and Neinhuis, 1997). In contrast, decreasing the contact angle should lead to enlarged values of adhesion (hydrophilic surfaces).

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65°

=1

CA

13.3 Contact angle of a water droplet on a superhydrophobic surface.

The contact angle of TiO2-coated materials also depends considerably on surface morphology, especially on average roughness. The deposition of photocatalytic thin films with controlled porosity can significantly improve photocatalytic efficiency (Hashimoto et al., 2005). In 1995 it was found that, besides the photocatalytic properties of TiO2, it demonstrates an intrinsic photo-induced surface super-hydrophilicity. This is one of the unique properties of TiO2 materials. Depending on processing techniques and chemical composition, a given TiO2 surface can have a more super-hydrophilic and less photocatalytic character, or vice versa (Fujishima et al., 2000). When irradiated by UV light, the water adsorbed on this semiconductor material spreads forming a thin film instead of a water droplet. Other materials do not possess these properties. Strontium titanate surfaces, which have a photocatalytic oxidation power comparable to TiO2, are not superhydrophilic under UV irradiation. A WO3 surface possesses photo-induced surface super-hydrophilicity conversion but does not show photocatalytic activity (Fujishima and Zhang, 2006). It has been proposed that the mechanism behind this property of TiO2 is related to the reconstruction of hydroxyl groups under UV irradiation (Fujishima et al., 2000). Photo-excited electrons are captured by molecular oxygen and the holes diffuse to the TiO2 surface where they are trapped by lattice oxygen atoms. The energy between the Ti atoms and lattice oxygen is weakened by hole trapping. Another adsorbed water molecule breaks this bond, forming a new hydroxyl group (Fujishima et al., 2000). The successive dissociative adsorption of water induces the trapping of these

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hydroxyl species leading to the photogeneration of a hydrophilic domain (size of about 10 nm) (Sakai et al., 2003). Given the unstable state of this surface, the photogenerated bound hydroxyl groups desorb gradually and the surface returns to the initial state demonstrating less hydrophilic behavior. In the case of photo-inducedsuper-hydrophilicity, the photo-electrons are trapped by tetravalent titanium ions Ti4+ through the formation of Ti3+ ions that are then oxidized by oxygen (Shapovalov, 2010). The following reaction equation describes the entire process (Fujishima et al., 2000): hυ ≥ E , k

bg 2 ⎯⎯⎯⎯ ⎯⎯⎯ → ≡ Ti − OH HO − Ti ≡ ≡ Ti − O − Ti ≡ + H 2 O ← ⎯

dark , Δ , k−2

[13.8]

where k2 and k−2 are the rate constants of the hydrophilic transformation for direct (under UV radiation) and inverse processes (without UV radiation), respectively. In this sense, TiO2-based surfaces can maintain their hydrophilic properties indefinitely as long as they are irradiated by UV light (Fujishima et al., 2000). If one recalls the cleaning action of a stream of water, TiO2-based surfaces could be, in theory, anti-staining and anti-soiling without the need of any chemical detergents. Furthermore, the application of TiO2-based materials can lead to the creation of anti-fogging surfaces which are particularly important when applied to glasses and mirrors such as vehicle side-view mirrors. The fogging of mirrors and glass surfaces appears due to the cooling of water steams that lead to the formation of many small water droplets which scatter light (Augugliaro et al., 2010). Super-hydrophilic surfaces prevent the formation of such droplets through the creation of a uniform water film which prevents the surface becoming unclear (Hashimoto et al., 2005). Moreover, super-hydrophilic surfaces are averse to absorbing organic liquids and can easily be cleaned with water. This feature is highly desirable in kitchens and toilets as well as in surfaces exposed to more polluted environments, e.g. in urban areas. Additionally, multilayer thin films can be used to combine the exceptional TiO2 properties with other technologies and/or materials (Augugliaro et al., 2010). As an example, the Leonardo Fioravanti (former Ferrari designer), presented novel car windows that do not require widescreen wipers at the Geneva Motor Show. The Hidra’s front and rear panels are composed of a four-layer film that imparts self-cleaning properties to the glass under any environmental conditions. In the layered structure, a super-hydrophilic coating of TiO2 was applied to avoid water droplets formation and to photodegrade volatile pollutants. Since TiO2 absorb UV light, the panels will also behave as a shield against this fraction of the solar spectrum (Augugliaro

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et al., 2010). In fact, discovering the photo-induced-super-hydrophilicity of TiO2 has markedly broadened the application range of titanium-based materials. There is a wide range of approaches used to deposit TiO2 layers on glass and polymer substrates, such as physical and chemical vapor deposition, sol–gel and dip-coating techniques, among others (Sabate et al., 1992; Chen et al., 1999; Zheng et al., 2003; Carneiro et al., 2005). TiO2 layers have generally required further heat treatments at relatively high temperature (Al-Jufairi, 2006). The production of crystalline layers at room temperature is preferable. TiO2 occurs in three crystalline phases (rutile, anatase and brookite), among which anatase is believed to be the most efficient photocatalyst during chemical reactions (Carneiro et al., 2005). Liquidphase deposition techniques based on sol–gel methods can be used to prepare high quality crystalline TiO2 nanoparticles-based layers at low production costs. The large-scale application of TiO2-based materials is limited by the need for an external UV excitation source (Turchi and Ollis 1990; Tavares et al., 2007). TiO2 photocatalytic efficiency is low under outdoor solar light irradiation. Most research has focused on nanosized TiO2 to improve light absorption through the high surface-to-volume ratio of nanograins (Shannon, 1976). In addition, increasing the generation rate of charge carriers is one strategy to enhance photocatalytic activity. Nanoparticles, with their increased surface area, can provide surface states within the band-gap to effectively reduce it (Zheng et al., 2003). Moreover, the addition (doping) of foreign ions (Fe+3, Cr+3or Pd+4) has been reported as a promising strategy to increase the visible light absorption of TiO2 materials (Carneiro et al., 2005). Figure 13.4 shows the optical transmittance spectra of TiO2 coatings deposited on glass substrates by DC reactive magnetron sputtering. It can be seen that, for Fe-doped TiO2 coatings, the absorption edge shifts to long wavelengths. This red-shift has been attributed to the excitation of 3d electrons of Fe3+ to the conduction band (Asiltürk et al., 2009). In fact, the main purpose of Fe doping is to extend the light absorption edge in order to make use of the majority of the ambient light spectrum. Another important technological application of TiO2-based materials (applied as nanoparticles or thin films) is its use as an anti-microbial agent across larger surface areas. The presence of dust, soil and spilled fluids in association with suitable temperature and humidity provides good conditions for a rapid multiplication of microorganisms. The application of TiO2-based coatings to tiles placed in surgical facilities could substantial decrease the amount of microorganisms present in materials in close contact to patients. The anti-bacterial mechanism of TiO2 is based upon the following:

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Transmittance (%)

90

60

30 Undoped TiO2 Fe-doped TiO2 0 300

400

500 600 700 Wavelength (nm)

800

900

13.4 UV-vis specta of undoped and Fe-doped TiO2 coatings produced by DC reactive magnetron sputtering.

• •

the TiO2 photocatalyst generates highly reactive species that attack the outer membrane of cells; an intrusion of copper ions into cytoplasmatic membrane kills microorganism’s cells.

In medical textiles, for instance, anti-microbial agents can be used to prevent staining and rotting of fibers, decrease unpleasant odors and the health risks associated with microbial growth. This mechanism of photocatalytic deactivation of microorganisms requires more time in indoor conditions than in outdoor conditions. However, the anti-bacterial properties of TiO2 induced by its photocatalytic capacity are strongly enhanced by fluorescent lamps (even with weak UV light) using either silver or copper metal dopants, both of which are harmless to human beings. An anti-bacterial effect can be observed on TiO2 doped with silver (Ag) (Hashimoto et al., 2005).

13.4

New and smart applications of TiO2 coatings

TiO2-based materials have been used to provide self-cleaning, anti-bacterial and anti-fogging functions based on photo-induced hydrophilicity and decomposition photoreactions. It is important to note that these functions do not require chemical compounds: theoccurrence of sunlight and rainwater are sufficient. In this sense, TiO2-based materials are environmentally friendly.

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13.4.1 Photocatalytic oxidation of NOx gases applied to constructive materials Industrial activities and road traffic are the main causes of the emission of pollutant gases such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). The health costs related only with road traffic air pollution represent 0.9–2.7% of the gross domestic product (GDP) in France, for example (Sommer et al., 1999). This pollution also reduces the appearance and durability of building materials. Inorganic photocatalysts, such as TiO2, have proven to be a relatively cheap and effective way to remove toxic organic compounds and pollutant gases from air and aqueous environments (Chen et al., 1997; Bilms et al., 2000; Hashimoto et al., 2000; Nakamura et al., 2000). An important and smart application of TiO2 photocatalysis is its use in construction and building materials. Given their large surface areas, buildings can even be used to promote the removal of pollutant gases from the surrounding air. Under UV irradiation, a TiO2 electron located in the valence band is promoted to the conduction band, creating an electron-hole pair (see Eq. 13.1) which can participate in a variety of redox reactions. Harmful NOx gases can be oxidized to nitrates on TiO2 surfaces activated by UV radiation (Oosawa and Graetzel, 1988). Since hydroxyl radicals (OH•) and super-oxide ions (O2−) are highly reactive species, they can react with nitrogen oxides to form nitrates. These nitrates can then be easily consumed and recycled by plants. Dalton et al. (2002) suggested that the NOx removal process involves the following steps: 1. Oxidation using hydroxyl radicals: OH• NO( g ) + 2OH(•ads) → NO2(ads) + H 2 O(ads) or NO2(ads,g ) + OH(•ads) → NO3−(ads) + H(+ads) 2. Oxidation using active oxygen: O2− O−

2 ( ads ) NOx (ads) ⎯⎯⎯ → NO−3(ads)

3. Reaction with Ti–OH via disproportion 3NO2 + 2OH − → 2 NO−3 + NO + H 2 O 4. Removal of [HNO3] complex from a material surface by water [HNO3](ads)→HNO3(aq) Figure 13.5 summarizes the overall reactions. Currently, the photocatalysis process requires a large amount of both light energy and photocatalyst material to remove pollutants from the

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Nanocoatings and ultra-thin films Sunlight (UV radiation)

NO

NO

NOx

NOx

NO2

NO2 OH•

•

O2–

OH

•

•

O2–

TiO2

NO3–

TiO2 NO3–

13.5 Schematic illustration showing NOx photodegradation process.

Activation of TiO2 Inserted glass cullets

TiO2-modified bituminous formulation

13.6 Pathways of light and activation of TiO2 in road pavement materials using glass as aggregates. (Adapted from Chen, and Poon, 2009).

surrounding environment. This has not yet made practical applications feasible. A suggested application is the inclusion of TiO2 micro/nanoparticles and recycled glass cullets in traditional road pavement material formulations. Since photocatalytic activity depends on the available electron/hole photo-induced pairs on surface of TiO2 micro/nanoparticles, the option of adding recycled glass cullets onto road pavement formulations should promote an in-depth conduction and entrapment of light, increasing NOx photodegradation efficiency. In theory, solar light would be carried to a greater depth, activating the TiO2 within the inner part of the surface layers as well as on the surface. This both meets recycling objectives and reduces pollution. The proposed mechanism for photocatalytic activity enhancment by inclusion of glass cullets is illustrated in Fig. 13.6.

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13.4.2 Treatment of soils polluted by volatile organic compounds Environment pollution of water, air and soil is becoming an increasingly serious problem. Hashimoto et al. (2005) have suggested a system for purification of soils polluted by VOCs based on the application of TiO2 photocatalysis activated by solar light. This purifying system employs a photocatalytic sheet, made of grooved paper, which has a TiO2 powder adsorbed onto activated carbon powder, as shown in Fig. 13.7. This sheet covers the polluted soil. The covered soil needs to be heated to volatilize the pollutant gases, that are subsequently captured by adsorption on activated carbon incorporated in the sheet material. Meanwhile, the TiO2 photocatalyst completely decomposes the pollutants by a photo-activated redox reaction. This purification method promises a sustainable form soil de-pollution.

13.4.3 Efficient energy-saving technology: water evaporation using hydrophilic surfaces The level of energy consumption in high density urban environments causes an inevitable temperature increase in the urban environment, the so-called heat island phenomenon. This can increase the heat absorbed by individual buildings which need then to be cooled by air-conditioning to maintain an acceptable living environment. Hashimoto et al. (2005) have suggested a system (illustrated in Fig. 13.8) which involve sprinkling water continuously onto building facades previously coated with a TiO2 photocatalyst.

VOCs photocatalytic degradation VOCs

TiO2 corrugated sheet Polluted soil

13.7 Purification system for polluted soils using solar energy and photocatalytic sheets. (Adapted from Hashimoto et al., 2005)

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Hydrophilic surface Latent heat flux by water evaporation

Rain water reservoir

13.8 Energy-saving system using solar light and stored rainwater.

Under the action of solar light irradiation the TiO2-coated building surface becomes super-hydrophilic. This surface state means that even a small amount of water sprinkled on the building would be enough to form a water thin film that would cover the entire building facade. It is important to highlight that the surfaces of buildings are cooled via a thermodynamic process, by releasing a certain amount of heat that results from water evaporation. This smart process for cooling down the buildings would decrease the electricity consumed by conventional air conditioning.

13.5

Conclusions

Although photocatalysis research emerged about 80 years ago, it was not until 1960 that scientific research resulted in any commercial product using TiO2 as a photo-active material. However, heterogeneous photocatalytic oxidation by TiO2-based materials makes them an attractive option in comparison to other oxidizing contaminant processes. Currently, this semiconductor has been widely applied to produce surfaces with self-cleaning, de-polluting, anti-fogging and anti-microbial abilities. TiO2-based materials have been deposited, using different technologies, to tailor surface properties of dissimilar surfaces such as textiles, vehicle side-view mirrors, lamps, buildings and road paving. Among them, liquid phase deposition techniques based on sol–gel methods are particularly promising since they can be used to prepare high quality crystalline TiO2 nanoparticles-based layers at low production costs and on an industrial scale.

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Since their self-cleaning and de-pollution abilities arise from the intrinsic properties of TiO2-based materials (without the need of any chemical compound), they are environmentally friendly. Given their large surface areas, buildings and pavements can be used to promote the removal of pollutant gases (NOx, for instance) from the air, thus improving air quality and the population’s health.

13.6

References

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Chen L X, Rajh T, Wang Z and Thurnauer M C (1997), ‘XAFS studies of surface structures of TiO2 nanoparticles and photocatalytic reduction of metal ions’, J Phys Chem B, 101, 10688–10697. Chen C H, Kelder E M and Shoonman J (1999), ‘Electrostatic sol-spray deposition (ESSD) and characterisation of nanostructured TiO2 thin films’, Thin Solid Films, 342, 35–41. Dalton J S, Janes P A, Jones, N G, Nicholson J A, Hallam K R and Allen G C (2002), ‘Photocatalytic oxidation of NOx gases using TiO2: a surface spectroscopic approach’, Environ Pollut, 120, 415–422. Feng L, Li S, Li H, Zhang L, Zhai J, Song Y, Liu B, Jiang L and Zhu P (2002),’ Super-hydrophobic surfaces: from natural to artificial’, Adv Mater, 14, 1857– 1860. Fujishima A and Honda K (1972), ‘Electrochemical photolysis of water at a semiconductor electrode’, Nature, 238, 37–38. Fujishima A and Zhang X (2006), ‘Titanium dioxide photocatalysis: Present situation and future approaches’, Compt Rendus Chem, 9, 750–760. Fujishima A, Rao T N and Tryk D A (2000), ‘Titanium dioxide photocatalysis’, J Photochem Photobio C Photochem Rev, 1, 1–21. Fujishima A, Zhang X and Tryk D A (2008), ‘TiO2 photocatalysis and related surface phenomena’, Surf Sci Rep, 63, 515–582. Gagliardi M (2010), ‘Photocatalysts: Technologies and Global Markets’, bcc Research, available at: http://bccresearch.wordpress.com/2010/03/30/global-market-for-photocatalyst-products-to-reach-1-6-billion-by-2014/ (accessed April 2011). Hashimoto K, Wasada K, Toukai N, Kominami H and Kera Y (2000), ‘Photocatalytic oxidation of nitrogen monoxide over titanium (VI) oxide nanocrystals large sizes areas’, J Photochem Photobiol A Chem, 136, 103–109. Hashimoto K, Irie H and Fujishima A (2005), ‘TiO2 photocatalysis: a historical overview and future prospects’, Jpn J Appl Phys, 44, 8269–8285. Heller A (1995), ‘Chemistry and applications of photocatalytic oxidation of thin organic films’, Acc Chem Res, 28, 503–508. Hoffmann M R, Martin S T, Choi W and Bahnemannt D W (1995), ‘Environmental applications of semiconductor photocatalysis’, Chem Rev, 95, 69–96. Lia Y, Lib X, Lic J and Yinc J (2006), ‘Photocatalytic degradation of methyl orange by TiO2-coated activated carbon and kinetic study’, Water Res, 40, 1119–1126. LinsebiglerA L, Lu G and Yates J T (1995), ‘Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results’, Chem Rev, 95, 735–758. Mills A and Hunte S L (1997), ‘An overview of semiconductor photocatalysis’, J Photochem Photobiol A Chem, 108, 1–35. Mills A and Lee S K (2002), ‘A web-based overview of semiconductor photochemistry-based current commercial applications’, J Photochem Photobiol A Chem, 152, 233–247. Nakamura I, Negishi N, Kutsuna S, Ihara T, Suggihara S and Takeuchi K (2000), ‘Role of oxygen vacancy in the plasma treated TiO2 photocatalyst with visible light activity for NO removal’, J Molec Catal A Chem, 161, 205–212. Oosawa Y and Graetzel M (1988), ‘Effect of surface hydroxyl density on photocatalytic oxygen generation in aqueous TiO2 suspensions’, J Chem Soc Faraday Trans, 1(84), 197–205.

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Self-cleaning smart nanocoatings

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Osterloh F E (2008), ‘Inorganic materials as catalysts for photochemical splitting of water’, Chem Mater, 20(1), 35–54. Paz Y (2010), ‘Application of TiO2 photocatalysis for air treatment: patents’ overview’, Appl Catal B Environ, 99, 448–460. Ramirez A, Demeestere K, De Belie N, Mantyla T and Levanen E (2010), ‘Titanium dioxide coated cementitious materials for air purifying purposes: Preparation, characterization and toluene removal potential’, Build Environ, 45, 832–838 Sabate J, Anderson M A, Kikkawa H, Xu Q, Cervera-March S and Hill C G (1992), ‘Nature and properties of pure and Nb-doped TiO2 ceramic membranes affecting the photocatalytic degradation of 3-chlorosalicylic acid as a model of halogenated organic compounds’, J Catal, 134, 36–40. Sakai N, Fujishima A, Watanabe T and Hashimoto K (2003), ‘Quantitative evaluation of the photoinduced hydrophilic conversion properties of TiO2 thin film surfaces by the reciprocal of contact angle’, J Phys Chem B, 107, 1028–1035. Schrank S G, Jose H J and Moreira R F M (2002), ‘Simultaneous photocatalytic Cr(VI) reduction and dye oxidation in a TiO2 slurry reactor’, J Photochem Photobiol A Chem, 147, 71–76 Shah S I, Li W, Huang C P, Jung O and Ni C (2002), ‘Study of Nd3+, Pd2+, pt4+, and Fe3+ dopant effect on photoreactivity of TiO2 nanoparticles’, PNAS, 99, 6482–6486 Shannon R D (1976), ‘Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides’, Acta Crystallogr A, 32, 751–767. Shapovalov V I (2010), ‘Nanopowders and films of titanium oxide for photocatalysis: a review’, Glass Phys Chem, 36, 121–157. Sommer H, Seethaler R, Chanel O, Herry M, Masson S and Vergnaud J-C (1999), Health Costs due to Road Traffic-related Air Pollution – An impact assessment project of Austria, France and Switzerland, Bern, Federal Department of Environment, Transport, Energy and Communications Bureau for Transport Studies. Stamate M and Lazar G (2007), ‘Application of titanium dioxide photocatalysis to create self-cleaning materials’, Roman Tech Sci Acad, 3, 280–285. Tavares C J, Vieira J, Rebouta L, Hungerford G, Coutinho P, Teixeira V, Carneiro J O and Fernandes A J (2007), ‘Reactive sputtering deposition of photocatalytic TiO2 thin films on glass substrates’, Mater Sci Eng B, 138,139–143. TOTO Ltd (2011), How hydrotect Works, available at http://www.totousa.com/ Green/HowHydrotectWorks.aspx (accessed May 2011). Turchi S and Ollis D F (1990), ‘Photocatalytic degradation of organic-water contaminants – mechanisms involving hydroxyl radical attack,’ J Catal, 122, 178–192. Yu J, Zhao X and Zhao Q (2000), Effect of surface structure on photocatalytic activity of TiO2 thin films prepared by sol–gel method, Thin Solid Films, 379, 7–14. Zheng S K, Xiang G, Wang T M, Pan F, Wang C and Hao W C (2003), ‘Photocatalytic activity studies of TiO2 thin films prepared by r.f. magnetron reactive sputtering’, Vacuum, 72, 79–84.

© Woodhead Publishing Limited, 2011

Index

adhesion, 59 aerospace engineering conventional coating technologies and smart nanocoatings for corrosion protection, 235–70 aluminium and its alloys, 243–4 anodising coatings, 253–7 applications of nanomaterials, 266–70 corrosion in aeronautical structures, 235–6 corrosion of magnesium alloys, 244–5 detection of corrosion and mechanical damage, 259–61 factors influencing corrosion, 241–3 functional nanocoatings, 258–9 future trends, 270 pretreatments, 247–53 protective coatings in aerospace engineering, 246–7 self-healing coatings, 261–6 types of corrosion in aircraft, 236–41 air/knife coating, 11 Alclad, 244 aluminium see also Alclad corrosion, 243–4 Amontons-Coulomb law, 357 Amontons’ law, 388 AMS2488, 382

analytical methods electrochemical methods, 132–40 cathodic stripping for mechanically polished copper specimens, 133 dependence of current density peak and potential peak on potential scan rate for copper specimens, 134 electrical equivalent circuit used to model the impedance data, 136 electrochemical impedance spectroscopy, 133–9 experimental data vs impedance calculated using KK relationships, 137 Nyquist plot for AISI 316L stainless steel, 135 organic coatings porosity, 140 parameters used in the simulation of impedance data, 136 potentiodynamic potential/current measurements, 132–3 water absorption in organic coatings, 139–40 nanocoatings and ultra-thin films, 131–53 spectroscopic, microscopic and acoustic techniques, 145–53 glow discharge optical emission spectroscopy, 150–1

414 © Woodhead Publishing Limited, 2011

Index infrared, Raman and Mössbauer spectroscopies, 145–7 ion scattering, Rutherford backscattering and secondaryion mass spectroscopy, 148–50 scanning acoustic microscopy and Kevin probe force microscopy, 152–3 scanning electron microscopy and transmission electron microscopy, 151–2 x-ray diffraction, 147–8 surface-sensitive analytical methods, 140–5 characterisation by AFM, 141 specular reflectance infrared spectroscopy characterisation, 143–5 XPS study of corrosion protection, 142–3 anodic aluminum oxide, 292 anodisation, 63 anodising, 6, 253–7 electrochemical chracterisation, 255–6 fatigue properties, 256–7 S-N plots for Al 2024-T3, 257 film structures, 254–5 micrograph of anodised Al 2024, 256 TEM micrographs of aluminium, 255 anti-microbial packaging, 215–16 anti-Stokes scattering, 146 antistatic packaging, 220–1 architectural glass, 182–95 dynamic smart glazings, 188–94 activated photoelectrochromic device, 192 composition of electrochromic glazing, 189 composition of glass laminate with liquid crystals or suspended particles, 192 electrochromic glazings, 188–9

415

gasochromic electrochromic device composition, 191 gasochromic-electrochromic glazings, 190–1 glazing composition with two complementary electrochromic layers, 189 light control and thermal imaging glazings, 193–4 liquid crystal and suspended particle glazings, 192–3 photochromic-electrochromic glazing devices, 191–2 photovoltaic-powered electrochromic device, 190 photovoltaic-powered electrochromic glazings, 190 thermally-activated glazings, 193 glass surface protections, 194–5 water droplet contact angle on hydrophilic and hydrophobic surface, 195 spectral transmittance and reflectance clear float and low iron and low-e glass, 186 low-e glass with single and double Ag layer, 187 low-e glass with single and triple Ag layer, 187 spectrally selective glass, 183–8 glass with low-emissivity coatings, 184–8 low-e coating layer, 185 optical properties and emissivity of glass, 183–4 ASTM 2625, 382 ASTM B449-93, 247 atmospheric pressure CVD, 72–3 atom transfer radical polymerisation, 102–3, 105 atomic force microscopy, 32, 308–9 atomic layer deposition, 71 automotive, 363–4 piston skirt damage due to severe scuffing, 364

© Woodhead Publishing Limited, 2011

416

Index

Bayresit, 210 benzophenone, 102 biomaterials, 366 biomedical implants industry, 174–5 most common techniques for hydroxyapatite coatings formation, 176 bismuth telluride, 344 bit patterned media, 318 Boegel, 252 ‘bond coat,’ 138 bottom-up method, 106 Bragg’s law, 147 Brasher–Kingsbury empirical relationship, 140 Brewster angle microscopy (BAM), 27 capillary force lithography, 301 carbon nanotubes, 217, 221 catalysed photoexcitation, 400 cationic polymerisation, 295 cerium, 248–9 chalcogenide, 343 chemical conversion coatings, 159–61 chromate and chromate-free conversion coatings, 160–1 conversion coatings by hydrothermal treatment, 160 chemical liquid deposition, 63 chemical shift, 142, 147 chemical solid growth, 63 chemical vapour deposition, 6–7, 71–4, 336, 373–5 chemisorption, 82 chromate conversion coating, 5–6, 152–3, 247–8 chrome-free conversion coatings, 6 chromic acid anodising, 254 chromium-free inhibitors, 248–50 chromogenic glazings, 183 cluster-beam PVD, 68 CMOS image sensors, 318–19 coating capacitance, 139 coating technologies advanced polymers and fillers, 17–18 conductive polymers, 17–18 fillers, 18

hyperbranched polymers, 17 organic–inorganic hybrid polymers, 17 water-soluble paint, 18 chemical and physical vapour deposition, 6–7 chemical vapour deposition, 6–7 physical vapour deposition, 7 coating processes developments, 18–20 conversion coatings, 5–6 anodising, 6 chromate conversion coating, 5–6 chrome-free conversion coatings, 6 current and advanced for industrial applications, 3–20 electro- and electroless chemical plating, 4–5 electrochemical plating, 4–5 electroless chemical plating, 5 new composite and powder coatings, 16–17 composite coatings, 16 powder coatings, 16–17 new lightweight materials, 12–13 other coating techniques, 10–12 air/knife coating, 11 curtain coating, 12 dip coating, 12 gravure coating, 10–11 knife over roll coating, 11 Meyer rod coating, 11 roll-to-roll coating, 11 slot/die and slot/extrusion coating, 12 sol–gel coatings, 10 spin coating, 10 spray coating, 7–10 cold spraying, 9 high-velocity oxygen fuel spraying, 8 plasma spraying, 8–9 thermal spraying, 8 vacuum plasma spraying, 9 warm spraying, 9–10

© Woodhead Publishing Limited, 2011

Index trends in coatings, 13–16 environmentally friendly coatings, 13 micro- and nanocapsule-based coatings, 14–15 nanocomposite coatings, 15–16 self-assembling molecules, 13 self-cleaning coatings, 14 coatings see also nanocoatings advanced technologies for automotive and aerospace industries, 162–70 modelling and computer simulations, 169–70 powder coating, 168 self-cleaning coatings, 167–8 ‘smart’ multifunctional coatings, 163 ‘super’-hard coatings, 164–7 thermal barrier coatings, 169 transparent coatings, 168 biomedical implants industry, 174–5, 176 conventional and advanced for industrial applications, 159–76 conventional technologies for automotive and aerospace industries, 159–62 chemical conversion coatings, 159–61 organic and inorganic coatings, 161–2 thermal-sprayed coatings, 162 electronics and sensor industry, 171–3 electronic nanodevices, 172–3 photovoltaic surfaces, 173 sensors, 171–1 most common techniques for hydroxyapatite coatings formation, 176 packaging applications, 170–1 coatings for food and pharmaceutical industries, 170 coatings in the paper industry, 170–1

417

paints and enamels industry, 173–4 coefficient of friction, 388 cold spraying, 9 ‘colloid probe,’ 141 combustion chemical vapour deposition, 73 complex non linear least squares (CNLLS) procedure, 135 compliance packaging, 206 composite coatings, 16 conductive polymers, 17–18 conductors, 58 controlled radical polymerisation, 102–6 conventional radical polymerisation, 92, 93–4, 102 conversion coatings, 5–6 anodising, 6 chromate conversion coating, 5–6 chrome-free conversion coatings, 6 conversion electron Mössbauer spectra, 147 corrosion, 236 aeronautical structures, 235–6 aluminium and its alloys, 243–4 anodising coatings, 253–7 electrochemical chracterisation, 255–6 fatigue properties, 256–7 film structures, 254–5 applications of nanomaterials, 266–70 deicing process, 268 XTEM bright field images of TaSi2–Si3N4 nanocomposite coating, 269 coating technologies and smart nanocoatings, 235–70 future trends, 270 factors, 241–3 susceptibility of metallic materials, 243 functional nanocoatings, 258–9 self-healing effect, 258 magnesium alloys, 244–5 nanocoatings for detection and mechanical damage, 259–61

© Woodhead Publishing Limited, 2011

418

Index

pH sensing coatings on aluminium alloys, 260 pretreatments, 247–53 chromate conversion coatings, 247–8 chromium-free inhibitors, 248–50 magnesium-rich primers, 252–3 sol–gel coatings, 250–2 protective coatings in aerospace engineering, 246–7 schematic of aerospace coating system, 246 self-healing coatings, 261–6 electrochemical impedance spectra of AA2024substrates, 262 scanning vibrating electrochemical technique, 265 types of corrosion in aircraft, 236–41 CFRP in Boeing 777 design, 240 fastener joint, 238 optical micrograph of scribe and filament, 237 corrosion fatigue, 241 crevice corrosion, 238 ‘critical brush density,’ 30 critical nucleus, 108 curtain coating, 12 deep drilling tools, 365–6 rock drill with cemented carbide buttons, 366 deformation, 360 Derjaguin–Muller–Toporov theory, 358 diamond-like carbon, 293–4 1,2-dichloroethane, 333 dielectric layers, 58 diffuse reflectance technique, 144 dip coating, 12 dip-pen nanolithography, 308 discrete track recording, 318 doping, 342 ‘double-pass transmission,’ 144 DS4 tester, 387 Durethan, 214 dynamic smart glazings, 188–94

edge lithography, 309–11 edible packaging, 220 electret, 303 electrochemical impedance spectroscopy, 133–9, 255–6 Bode diagrams for aluminium, 257 electrochemical liquid growth (ECLG) method, 63 electrochemical plating, 4–5 electrochromic glazings, 188–9 electroless chemical plating, 5 electron acoustic images, 152 electron-beam evaporation, 65 electron-beam evaporation PVD, 65–6 electron beam lithography, 283 electron spectroscopy for chemical analysis, 161 electronic packaging, 206–8 electronics nanoimprint lithography, 280–321 applications, 317–20 combined nanoimprint approaches, 315–17 edge lithography, 309–11 extension of soft NIL, 301–7 lithography techniques and fundamentals, 280–6 NIL for 3D patterning, 311–14 photo-assisted nanoimprinting, 291–6 scanning probe lithography, 307–9 soft NIL, 297–301 thermoplastic and laser-assisted nanoimprint lithography, 286–91 electroplating, 375 Elektron, 245 encapsulation, 218–19 ‘end-grafted polymers’ see ‘polymer brush’ energy dispersive x-ray spectroscopy (EDS), 148 environmentally friendly coatings, 13 equilibrium adsorption, 80 ethylene tetrafluoroethylene, 287 EVG, 318 exfoliation, 239

© Woodhead Publishing Limited, 2011

Index extreme UV lithography, 282–3 extrusion, 12 F-NAD, 161–2 fatigue tests, 256–7 field-effect transistor, 311, 332 filiform corrosion, 237–8 fillers, 18 fluorine-modified polymers, 161 focused ion beam, 283 food packaging, 205–6 fourier transform infrared Raman spectroscopy, 146 Fresnel equations, 145 fretting, 365 fretting corrosion, 241 friction, 356–8 functional graded nanocoatings, 57–60 gallium nitride, 344–51 AFM images, 347 CL spectra from dislocation cluster, 350 SEM images, 346, 348 SEM vs monochromatic μ-CL images, 349 galvanic corrosion, 239 gasochromic-electrochromic glazings, 190–1 GENOCURE, 296 geometric hindered factor, 213 glass surface protections, 194–5 glass transition temperature, 290 glow discharge optical emission spectroscopy (GDOES), 150–1 grafting density, 83–4 ‘grafting from’ approach, 92 ‘grafting through’ method, 106 ‘grafting to’ approach, 89–91 graphene 2D structures, 331–41 graphene FETs in the electrolyte solution, 341 HRTEM images of graphene membrane, 335 microfabrication steps, 333

419

morphological and structural analyses of the ZnO–G HAs, 338 optoelectronics, 339 graphene nanoribbons, 333 graphene oxide, 336 graphene sheets, 221 graphene transparent conducting films, 339–40 graphite fibres, 239–40 graphite particles, 221 gravure coating, 10–11 Greenwood–Williamson theory, 357–8 Hall–Petch relationship, 367–8 heterogenous nucleation, 108 hexagonal boron nitride phase, 342 high barrier packaging, 209–15 models of aligned mono-disperse flakes in periodic arrangement, 212 oxygen permeability as function of water vapour barrier properties, 210 oxygen scavenging materials, 214–15 samples of oxygen scavenging materials, 215 high ordered pyrolytic graphite, 47 high velocity oxy-fuel spraying, 8, 148, 162 holographic patterning, 312 homogenous nucleation, 108 hot embossing see thermal nanoimprint lithography Hydrotect, 398–9 8-hydroxiquinoline, 266 hydroxyapatite, 174, 175 hyperbranched polymers, 17 hyperfine splitting, 147 immersion lithography, 282 Imprio, 318 ‘in situ polymerisation,’ 209 in-situ tribometry, 387 indium tin oxide, 339 infrared spectroscopy, 145–6 InMat Nanocoatings, 211

© Woodhead Publishing Limited, 2011

420

Index

inorganic coatings, 161–2 interfacial shear strength, 358–9 intergranular corrosion, 239 International Technology Roadmap for Semiconductors, 282 intimate contact area, 359–60 ion beam milling, 194 ion implantation process, 68 ion-plating PVD, 68 ion scattering spectroscopy, 148–9 irreversible adsorption, 80 isomer shift see chemical shift Johnson–Kendall–Roberts contact theory, 358 Kevin probe force microscopy (KPFM), 152–3 Kirchhoff’s law, 184 knife over roll coating, 11 Kramers–Kronig test, 137, 145 Langmuir monolayer, 24 Langmuir–Blodgett assembly, 334 Langmuir–Blodgett film, 383 Langmuir–Blodgett technique, 87, 92 Langmuir–Blodgett–Kuhn multilayers, 32 Langmuir–Schaefer method, 27 Langmuir–Schaefer monolayers, 32 laser ablation PVD, 66 laser-assisted direct imprint, 290–1 laser-induced chemical vapour deposition, 308 lasercarb coating, 380 lateral force microscopy, 383 laws of friction, 357 layer-by-layer deposition, 87, 264 layered double hydroxides, 211, 262–4 light control and thermal imaging glazings, 193–4 light-emitting diodes, 193 linear potential sweep, 132 ‘liquid condensed phase,’ 27 liquid crystal and suspended particle glazings, 192–3 ‘liquid expanded phase,’ 26

‘liquid glass,’ 194 lithography techniques, 280–6 nomenclature of methods, 281 process steps, 285 lotus effect, 14 low energy ion scattering, 149 low pressure CVD, 72–3 magnesium alloys, 244–5 magnesium-rich primers, 252–3 matrix-assisted laser desorption/ ionisation, 66 matrix-assisted pulsed-laser evaporation, 66 mean surface roughness, 141 melt mixing process, 209 2-mercaptobenzothiazole, 266 metal corrosion, 235 metal-organic chemical vapour deposition, 365 Meyer rod, 11 Meyer rod coating, 11 micro-based coatings, 14–15 microcontact printing, 297, 298 microelectromechanical systems, 359 micromachining, 307 micromolding in capillaries, 300 microtribometers, 363 sample, 364 MIL-DTL-5541, 247 molecular beam epitaxy, 311 molecular-beam epitaxy PVD, 67–8 molybdates, 249 montmorillonite, 211 Mössbauer absorption spectrometry, 147 Mott–Schottky plots, 139 nanocapsule-based coatings, 14–15 nanoclay particles, 220 nanocoatings and ultra-thin films analytical methods, 131–53 electrochemical methods, 132–40 spectroscopic, microscopic and acoustic techniques, 145–53

© Woodhead Publishing Limited, 2011

Index surface-sensitive analytical methods, 140–5 architectural glass, 182–95 dynamic smart glazings, 188–94 glass surface protections, 194–5 spectrally selective glass, 183–8 chemical and physical vapour deposition methods, 57–75 chemical vapour deposition based technologies, 71–4 D-gun spray deposition of functional gradient coating from titanium and hydroxyapatite, 70 future trends, 74–5 paradigm of functional graded layer-by-layer coating fabrication, 60–1 physical vapour deposition based technologies, 63–71 substrate preparation for ultrathin films and functional graded nanocoatings, 57–60 typical installation diagram for PVD and CVD coating, 64 corrosion protection in aerospace engineering, 235–70 aluminium and its alloys, 243–4 anodising coatings, 253–7 applications of nanomaterials, 266–70 corrosion in aeronautical structures, 235–6 corrosion of magnesium alloys, 244–5 detection of corrosion and mechanical damage, 259–61 factors influencing corrosion, 241–3 functional nanocoatings, 258–9 future trends, 270 pretreatments, 247–53 protective coatings in aerospace engineering, 246–7 self-healing coatings, 261–6 types of corrosion in aircraft, 236–41 fabrication methods, 61–3

421

approximate classification scheme of nanocoating technologies, 62 packaging applications, 203–22 anti-microbial packaging, 215–16 antistatic packaging applications, 220–1 future trends, 222 high barrier packaging, 209–15 nanomaterials in packaging, 208–9 nanosensors in packaging, 216–18 nanotechnology solutions for packaging waste problem, 219–20 packaging as a drug carrier and for drug delivery, 218–19 regulation and ethical issues in new packaging industry, 221–2 surface-initiated polymerisation, 78–112 physisorption and chemisorption, equilibrium and irreversible adsorption, 79–87 properties and applications, 110–12 surface-bound polymer layers preparation, 87–110 nanocomposite coatings, 15–16 nanoimprint lithography, 284 3D patterning, 311–14 gold pattern fabrication, 314 electronics, 280–321 applications, 317–20 combined nanoimprint approaches, 315–17 edge lithography, 309–11 extension of soft NIL, 301–7 lithography techniques and fundamentals, 280–6 photo-assisted nanoimprinting, 291–6 scanning probe lithography, 307–9 soft NIL, 297–301 thermoplastic and laser-assisted nanoimprint lithography, 286–91 nanoimprinting in metal/polymer bilayer, 298 nanomaterials, 208–9

© Woodhead Publishing Limited, 2011

422

Index

Nanomiser device atomisation, 73 nanomolding in capillaries, 300 nanoreservoirs, 163 nanosensors, 216–18 nanoshaving, 308 nanostructure coatings challenges to establish scale up, 389–91 balance cost/performance ratio, 391 coat different sample sizes and nanostructure retention, 389–90 performance of laboratory tests and reliability analysis, 390–1 friction and wear applications, 367–82 deposition methods, 373–9 engineering materials, 379–82 structure-property relationships, 367–73 tribology, 355–91 friction and wear applications, 367–82 future trends, 391 tribological properties characterisation, 382–91 use of nanostructure coatings, 356–66 nanostructured composite, 209 nanostructured thin films 2-D arrays of colloidal spheres, 44–7 A2-D array fabrication methods at air–liquid interface, 45 amphiphilic molecules, 24–48 amphiphilic polymers, 28–36 block copolymers, 28–32 interfacial behaviour of ionic block copolymers, 31 π-conjugated polymers, 32–6 poly(p-phenylene) derivative chemical structure and π-A isotherm, 33 poly(phenylenevinylene) derivatives possible arrangement at the air–water interface, 34 polythiophene derivatives, 35

PS–b–PEO diblock copolymer surface morphological behaviour, 29 dendrons and dendrimers, 36–41 chemical structure of fourth generation poly(ethyl ether) with hexa(ethylene glycol) tail, 38 poly(amidoamine), 37–40 poly(benzyl ether), 36–7 poly(propylene imine), 40–1 PPI dendrimers, 41 tetra-dendronpoly(amidoamine) dendrimers, 39 Langmuir monolayer, 24–8 isotropic liquid film in trough with movable barrier, 25 Langmuir trough and other experimental techniques, 27 Langmuir–Blodgett rough being compressed, 25 recent researches, 28 surface pressure, 25 surface pressure isotherm, 26–7 surface pressure isotherm for Langmuir monolayer after reference 7, 26 metal/semiconductor nanoparticles, 41–4 TOPO-capped CdSe quantum dot, 43 nanotechnology, 208 solutions for packaging waste problem, 219–20 nanotransfer printing, 298 Nielson equation, 213 nitroxide mediated radical polymerisation, 102, 104–5 1-octadecanethiol, 43 optical proximity correction, 282 organic coatings, 161–2 organic electrochemical transistor, 34 organic thin film transistors, 296 organic–inorganic hybrid polymers, 17 Orowan mechanism, 369 oxalic acid, 254

© Woodhead Publishing Limited, 2011

Index oxide film, 254–5 oxygen scavenging materials, 214–15 packaging anti-microbial packaging, 215–16 antistatic packaging applications, 220–1 multilayer structure and chemical antistatic agent, 221 as a drug carrier and for drug delivery, 218–19 definition, 206 future trends, 222 high barrier packaging, 209–15 models of aligned mono-disperse flakes in periodic arrangement, 212 oxygen permeability as function of water vapour barrier properties, 210 oxygen scavenging materials, 214–15 samples of oxygen scavenging materials, 215 nanocoatings and ultra-thin films applications, 203–22 electronic packaging, 206–8 electronic packaging products, 207 examples of food packaging, 205 food packaging, 205–6 pharmaceutical packaging, 206 pharmaceutical packaging samples, 207 nanomaterials, 208–9 nanosensors, 216–18 radio-frequency identification, 218 nanotechnology solutions for packaging waste problem, 219–20 biodegradable food packaging, 219 principal objectives, 204 regulation and ethical issues in new packaging industry, 221–2 packaging applications, 170–1 food and pharmaceutical industries, 170 paper industry, 170–1

423

barrier-coated papers, 170–1 curtain coating, 171 patina, 132 3-pentadecylphenol, 30 pharmaceutical packaging, 206 phase-change random access memory, 318–19 phase changed materials, 267 photo-assisted nanoimprinting, 291–6 photocatalysis photocatalytic cleaning effect of TiO2-coated materials, 402–6 contact angle of water droplet, 403 UV-vis specta of undoped and Fe-doped TiO2 coatings, 406 processes, 399–402 band-edge energies of semiconductors, 400 photochemical excited TiO2 particle, 399 titanium dioxide, 397–9 photochromic-electrochromic glazing devices, 191–2 photoelectrochromic glazing, 191 photolithography, 282 photoresist patterns, 310 photovoltaic-powered electrochromic device, 190 photovoltaic-powered electrochromic glazings, 190 phyllo silicates, 18 physical liquid deposition, 63 physical solid deposition, 64 physical vapour deposition, 7, 63–71, 373–5 physisorption, 82 pitting corrosion, 238–9 plasma-assisted chemical vapour deposition, 372 plasma energy CVD, 73, 74 plasma spraying, 8–9, 162 poly(1,1-diethylsilacyclobutane)–b– poly(methacrylic acid), 30 poly(3-hexyl-thiophene), 149 polyamide 6, 210 poly(amidoamine), 37–40 poly(benzyl ether), 36–7

© Woodhead Publishing Limited, 2011

424

Index

polycarbonate, 288 polyetherimide, 288 polyethylene, 210 poly(ethyleneimine), 317 poly(hydrogenated–isoprene)–b–poly(styrenesulphonate), 30 polylactic acid, 210 polymer-assisted deposition (PAD) method, 69 ‘polymer brush,’ 87 polymer multilayer process, 60 ‘polymerisable complex (PC) route,’ 151 polymethyl glutarimide, 332 poly(methyl methacrylate), 288–9, 332 poly(N-dodecylacrylamide), 34 poly(N-isopropylacrylamide), 111 polypropylene, 210 poly(propylene imine), 40–1 polystyrene, 28 poly(styrene)-b-poly(ethylene oxide) (PS–b–PEO) copolymers, 28 poly(styrene–b–ferrocenyl silane), 30 polystyrene–block–poly(Nisopropylacryamide) (PS–b– PNIPAM) diblock copolymer, 29 polystyrene–b–poly(methyl methacrylate) (PS–b–PMMA) diblock copolymer, 29 poly(styrene)–b– poly(styrenesulphonate), 30 polystyrene–b–poly(vinylpyridine) (PS–b–PVP) copolymers, 30 polystyrene–graft–poly(ethylene oxide) (PS–g–PEO) copolymers, 29 powder coating, 16–17, 168 primary packaging, 204 projection lithography, 282–3 propyltrimethoxysilane, 210 quadruple splitting, 147 quantum dots, 43 quintuple layer, 343–4 radio-frequency identification, 218 Raman spectroscopy, 147

raster, 151 Rayleigh scattering, 146 reactive ion etching, 286–7 regioregular poly(3-hexylthiophene), 34 resistance-heated source evaporation, 66 reverse contact UV-NIL, 314 reversible addition–fragmentation chain transfer polymerisation, 102, 103–4 roll-to-roll coating, 11 Rutherford backscattering spectrometry, 149 ‘sacrificial initiator,’ 105 ‘sacrificial layer,’ 185 scanning acoustic microscopy, 152 scanning beam lithography, 282–3 scanning electron acoustic microscopy, 152 scanning electron microscopy, 151 scanning probe lithography, 307–9 Scheutjens–Fleer self-consistent mean-field theory, 111 secondary-ion mass spectroscopy, 149–50 secondary packaging, 204 self-assembled monolayers, 89 self-assembling molecules, 13 self-cleaning coatings, 14, 167–8 self-healing coatings, 261–6 electrochemical impedance spectra of AA2024substrates, 262 LDHs dual role in corrosion protection, 263 scanning vibrating electrochemical technique, 265 self-switching liquid crystal glazing technology, 193 semiconductor compounds, 341–4 semiconductors, 58 sensitized photoreaction, 400 sensor ultra-thin membranes, 330–51 gallium nitride, 344–51

© Woodhead Publishing Limited, 2011

Index graphene and 2D structures, 331–41 nanometer-thick membranes of semiconductor compounds, 341–4 silicone-modified polymers, 161 silver particles, 215–16 sliding test, 383 slot/die coating, 12 slot/extrusion coating, 12 ‘smart’ multifunctional coatings, 163 smart nanocoatings self-cleaning, 397–411 new and smart applications of TiO2-coated materials, 406–10 photocatalysis processes, 399–402 photocatalytic cleaning effect of TiO2-coated materials, 402–6 TiO2 photocatalysis, 397–9 smart packages, 222 Snell’s law, 144 soft lithography, 286 soft nanoimprint lithography, 297–301 conventional nanoimprint vs electrochemical nanoimprint vs surface charge lithography, 302 GaN-based mesostructures, 305 GaN nanowire merging triangular mesostructures, 306 SEM images from GaN layers subjected to PEC etching, 304 sol–gel coatings, 10, 250–2 sample structure, 252 silane deposition on metallic substrate, 250 solvent-assisted micromolding, 299–300 spectrally selective glass, 183–8 specular component, 144 spin coating, 10 spray coating, 7–10 cold spraying, 9 high-velocity oxygen fuel spraying, 8 plasma spraying, 8–9 thermal spraying, 8 vacuum plasma spraying, 9 warm spraying, 9–10 sputtering PVD, 67

425

Stefan–Boltzmann’s law, 184 step and flash imprint see UV nanoimprint lithography Stokes scattering, 146 stress-corrosion cracking, 241 structure-property relationships, 367–73 graded coatings, 370 historical trends in tribological coatings development, 368 nanocomposites, 370–2 TEM image of nc-TiN/a-C:H, 372 TEM image of WC–12Co coating, 372 smart adaptive coatings, 373 Au/MoS2/DLC/YSZ chameleon coating, 373 strengthening mechanisms, 367–9 hardness as function of grain size, 369 superlattice multilayer, 370 nanomultiayers of TiN/(Ti,Al)N, 371 sulphuric acid anodising, 254 ‘super’-hard coatings, 164–7 conventional coatings, 164–5 composite coating, 165 nitride/carbide.boride coatings, 164–5 nanocoatings, 165–6 nanocomposite coatings, 167 nitride nanocoatings, 166–7 oxide nanocoatings, 166 supersonic free-jet PVD, 69 surface corrosion, 236–7 surface-initiated polymerisation, 78–112 adsorption parameters, 82–5 free energy barrier and the strength of binding interaction, 82–3 grafting density and solvent nature, 83–5 polymer concentration in bulk, 83 nanocoating by ‘reverse grafting,’ 106–10 formation of polymer– nanoparticle complex, 109–10

© Woodhead Publishing Limited, 2011

426

Index

nucleation and phase formation, 106 schematic representation, 107 physisorption and chemisorption, equilibrium and irreversible adsorption, 79–87 adsorbed layers topologies, 86 comparative kinetic curves for amino mono-functionalised polystyrene chemisorption, 81 thermodynamics and kinetics, 79–82 topology of surface-bound layers, 86–7 polymer chemisorption processes, 88–106 controlled radical polymerisation, 102–6 conventional radical polymerisation, 92, 102 ‘grafting from’ approach, 92 ‘grafting through’ method, 106, 107 ‘grafting to’ approach, 89–91 properties and applications, 110–12 modified surfaces properties and applications, 111–12 surface-bound polymer layers physicochemical properties and comparison to bulk properties, 110–11 surface-bound polymer layers preparation, 87–110 ‘grafting from’ of unsaturated and cyclic monomers, 100–1 ‘grafting from’ systems based on controlled radical polymerisation, 95–9 ‘grafting from’ systems based on conventional radical polymerisation, 93–4 methods for preparation of surface linked polymer layers, 88 nitroxide-mediated surface grafting, 104 polymer physisorption techniques, 87–8

RAFT polymerisation, 103 surface ATRP propagation mechanism, 105 surface-bound polymer layers obtained by ‘grafting through’ approach, 107 surface-bound polymer layers obtained through ‘grafting to’ approach, 90–1 surface initiated conventional radical polymerisation, 102 surface polymerisation by ion-assisted deposition, 69 TE67, 387 tertiary packaging, 204 tethered density, 84 ‘tethered polymer layers’ see ‘polymer brush’ tetrabutylammonium hydroxide, 332 tetraisopropyl orthotitanate, 146 thermal barrier coatings, 138, 169 thermal evaporation PVD, 65 thermal expansion coefficient, 286 thermal nanoimprint lithography, 284–5 SEM images, 287 thermal-sprayed coatings, 162 thermal spraying, 8, 375, 377–8 thermally-activated glazings, 193 thermally grown oxide, 138 thermopolymerisation reaction, 290 thermotropic glazing, 193 ‘third body’ friction, 359 titanium, 174–5 titanium diboride, 269 titanium dioxide, 216 efficient energy-saving technology, 409–10 energy-saving system using solar light and stored rainwater, 410 new and smart applications, 406–10 NOx gases applied to construction materials, 407–8 pathways of light and activation of titanium dioxide, 408 schematic illustration, 408 photocatalysis, 397–9

© Woodhead Publishing Limited, 2011

Index photocatalytic cleaning effect, 402–6 soil treatment, 409 purification systems, 409 transmission electron microscopy, 151–2 transparent coatings, 168 tribology, 356 applications, 363–6 aerospace, 363 automotive, 363–4 biomaterials, 366 deep drilling tools, 365–6 mechanical components, 364 repair coatings, 366 turbines, 365 deposition methods, 373–9 coating deposition classification, 374 contact AFM images of WC–12Co, 378 growth pattern in PVD-deposited nc-TiN/a-Si3N4, 376 TEM image of cobalt-hardened gold electrodeposited nanostructured coating, 377 thermal spraying techniques, 378 engineering materials, 379–82 abrasion vs wear resistance of TiN/TaN multilayer coatings, 380 hardness of electrolytic Ni coating versus grain size and abrasion resistance, 379 mechanical and tribological properties of nanostructured coatings, 382 nanostructured HVOF WC–12Co– 2Al-coated vs chromium-plated crankshaft, 381 wear resistance comparison, 381 friction and wear applications, 367–82 structure-property relationships, 367–73 nanostructure coatings, 355–91 future trends, 391 tribological properties characterisation, 382–91

427

challenges in scale up establishment, 389–91 friction loops on etched dual phase steel, 386 friction loops showing topography and phase effects, 385 scale dependence, 387–9 schematic representation of friction mechanisms, 388 techniques for friction and wear characterisation, 382–7 wear track, 385 use of nanostructure coatings, 356–66 friction and wear mechanisms, 356–63 trioctylphosphineoxide, 43 Tupolev TU-134, 245 turbines, 365 ultra-hard materials, 15 ultra-thin films, 57–60 and nanocoatings analytical methods, 131–53 electrochemical methods, 132–40 spectroscopic, microscopic and acoustic techniques, 145–53 surface-sensitive analytical methods, 140–5 packaging applications, 203–22 anti-microbial packaging, 215–16 antistatic packaging applications, 220–1 future trends, 222 high barrier packaging, 209–15 nanomaterials in packaging, 208–9 nanosensors in packaging, 216–18 nanotechnology solutions for packaging waste problem, 219–20 packaging as a drug carrier and for drug delivery, 218–19 regulation and ethical issues in new packaging industry, 221–2 ultra-thin membranes sensor, 330–51 gallium nitride, 344–51

© Woodhead Publishing Limited, 2011

428

Index

graphene and 2D structures, 331–41 nanometer-thick membranes of semiconductor compounds, 341–4 ultrananocrystalline diamond , 167 UV nanoimprint lithography, 283–4, 285, 291–6 vacuum deposition methods nanocoatings and functionally graded multilayers, 57–75 vacuum monomer technique, 60 vacuum plasma spraying, 9 vanadates, 249 vapour phase epitaxy (VPE) process, 73 ‘velocity accommodation mode,’ 359 very low-temperature pyrolysis LPCVD, 73

Volta potential map, 153 vortical surface method, 47 warm spraying, 9–10 water-soluble paint, 18 wear, 360–3 mode types and sample case studies, 361 white graphite see hexagonal boron nitride phase Wien’s law, 184 Wilhelmy Plate, 27 X-ray lithography, 316 x-ray Mössbauer spectra, 147 x-ray powder diffraction method, 148 Zenner pinning, 369 zinc dialkyl dithiophosphate, 364 zirconium oxide coatings, 150

© Woodhead Publishing Limited, 2011